Does it make any more sense to have a power that deals cold and fire damage without the implement? Since you are channeling the power through the implement to get the enhancement bonus, I don't see why it shouldn't pick up a few characteristics of the implement on the way. Effect keywords still stay effects, even when damage is done, and damage keywords still stay damage even when there is an effect. No Fear damage, because fear is an effect, not damage. The disintigrate now has both a damage keyword and a fear keyword. The warlock is not scary, the wand makes the power have the fear keyword. Not exactly intuitive, but neither is magic, and that is what we are talking about.
Seems to me that all this does is make the max 4 dailies actually really good instead of just a 1.5 round speedbump. I'm OK with that at this point. I may change my tune with some more playtime, but now I'm OK. Your math also didn't include the bonus to saves that solos and elites get. Just from memory I think it is something like +6 and +2 respectively. That changes things. And with a 26 wis, you gotta be 21st level at that point. I'm OK with that level of power at that epic level.
The warlock being scary I was referring to was the doomsayer class ability (this isn't necessarily the case, I was just positing a possible fluff). The thing with disintegrate is, the ongoing damage is a save effect. So the fear keyword + warlock = lower chance of saving versus just a straight damage effect (because the warlock is scary, or whatever that class ability is attempting to model, and the disintegrate is also now doing a save end effect with a fear keyword, that happens to be just damage). I don't really think that makes much sense.
Solo's get +5 to saves and elites get +2 to saves. I took that into account. Without the +5 save bonus the average round time would be over 200 rounds (basically, its dead). With just the elite (+2) bonus the average round time would be something like 30 rounds. The reason I didn't include those numbers is that I think wasting that much resource to keep one guy out when you're fighting a ton of them isn't as awesome a tactical maneuver. When its a solo it is.
Equation I used: +5-2-8=-5 total mod on a solo save. This means he needs a 15 or higher (30% chance). Both saves need to succeed in order to get out. 9% chance per turn. .09^8=.47%. So it's 7 or 8 rounds (I was using a wisdom mod of 9 not 8 earlier for some reason, I fixed it now).
Being stunned for 8 rounds is not just "making those abilities a little better." I play wizard pretty much exclusively, and stun, even for one round on a solo is AMAZING. Thats one round where your entire team gets to wail on it for free, with combat advantage. Stun means no actions, no opportunity attacks, nothing. I understand that this (the 8 round stun) is a high level effect, but it will completely destroy any level appropriate (or even much higher level, as long a stun save end effect hits once) monster will be completely dead.
8 rounds of stun means the solo is dead. Obviously if someone hit with an unconscious effect it would be even worse. Unless there is some kind of unusual circumstance, 8 rounds of focus fire from a level equivalent party, with combat advantage, is going to kill even the huge hp solos.