As promised here are some various alternative solutions to whatever it is you're trying to accomplish with the exp change. These solutions can accomplish your goals WITHOUT screwing over 60% of your player base which imho should be avoided at all cost, we can't afford to lose what little p-base we have left in the name of 'balance'.

I spoke with you briefly about the goal of your change, and whaty ou wanted to accomplish. And some options I'll give you here can have various other effects but short and long term they tend to have much smaller side effects. Also my alternatives do not in any way indicate what I view is the perfect solution to darke it's just a few random thoughts I came up with in a very short period of time to accomplish what you want without negative side effects.

Your goal: To help out the non-haste guilds in regards to their exp intake as a result of the exp change. To bring them somewhat up to par with other guilds.

Factors to consider:
1: How do we do this without giving any unfair advantages to already existing high characters?

2: Is our goal to actually increase the actual C levels of non-haste guilds, or simply to give a benefit to what their existing C levels translate to in a gameplay environment?

3: If 2 = give a benefit, to what extent? Is it a pvp-benefit we wish to give to help them remotely compete in a small way, or is it a dev-point benefit, or a wield benefit or etc?

4: Before deiciding on the best solution, you have to determine if the C levels and speed of which those non-haste guilds are leveling is at your target goal, or how far away is it?

Solutions:
Instead of reducing exp or changing the way hasted combat gives exp which as you've already said has many negative side effects here are some options:

1: Give a X% exp bonus to any "hit" performed without haste on a character (You also need to determine if this bonus should apply to single wield only or if dual should be considered - provided both without haste of any kind including fighter innate haste)

Example: If each tick of combat exp an enchanter, or elementalist or etc earned was given a 50% exp modification that would be pretty significant. This theoretically should not make them earn more than say an arch mage with haste, as they get double hits. It would be closer than you think though given the use of Fatigue Points and hasters having to sit on ahealing rune while a non-haster would keep fighting.
***Other results of this. Combat levels for non hasted guilds would rise slightly, the time spent per level would be reduced. This would give a small bonus to those guild's overall levels which applies to spell resistance and distance magic *but very minimal*

What this would NOT do is change the PVP dynamic (if that is your goal). Sure the average enchatner/elem etc might have slightly more wield, they still will have huge disadvantage of 50% combt dev points per level, as well as 1 APR - chance to crit per level. Lower wield + fewer attacks - vastly disproportionate PVP. Perhaps in addition to this 50% bonus, a small critical bonus escalation (randomLY) on non-hasted hits might occur? This should not be a large bonus as it could have big effects, but just another way to make fighting a bit more interesting.

******************************
If your goal is to NOT increase the actual levels, because perhaps philosophically we like enchanters elems etc having lower C levels (and maybe rightly so) but what they can DO with them needs to be modified? If this is the case the solution is simple

Take the non-haste guilds and boost their DEV-Level % ratio slightly. Combat exp change is nothing more than adding Devs and a few hit points to players. So instead of 50% Devs per combat level, make them 75%, this gives them more wield-per-level, but since they are neither a hasted guild, nor a spellup-boosted combat guild this has virtually no impact other than raising their wield at any given level, which is the same as boosting their combat exp without giving them extra hit points.
*The 50% dev or 75% dev etc concepts were put in place as a way to balance combat in relation to a pure combat guild to a pure magic, or semi etc being at 75's, the problem is, the haste and spellup/skill bonus spells and abilities already accmoplish this pretty much without having to have those few non-haste guilds at 50%. (thieves elems enchanters). Make sense?

Again I've just barely hit the tip of the iceberg here and obviously any change is going to have many other subtle effects, but this is a place to start. At least these changes don't slap your lower players in the face and say yeah, we know you were hoping to build this character up but now you gotta spend 4x longer doing it than the guy next to you that happens to be killing you spent.

If this is confusing or difficult to understand talk to me in game (hannibal/onslaught/dorne/edge) Again these solutions aren't perfect, to me the only true way to fix darke is a huge balance project with an inevitable p-wipe and a very large philosophical game design change, but in lieu of that let's make intelligent changes that don't drive off the player bse

In summary. Yes I've ignored the whole point you may be thinking of if you think "there is too much combat exp being earned in general" That issue can easily be addressed to AFTER these other changes by making a very simple change in the formula which translates wield - to crits

It's an array/formula that says at 200 wield you tend to send this crit table to a b c d x % of time etc, that can always be fundamentally changed with minimal effort. So in effect we CAN "bring everyone up to a high level" and at the same time, adjust what that high level means in regards to game play and PVP. easy eh?

Onslaught