Top End won't be sacraficed as much as some say. More power everywhere. Being more powerful it pulls through the revs quicker. This allows it to pull taller gearing also. You will also find you will be able to lug the motor more on hill climbs & the technical snot. Jetting & gearing is all you will really need to sort.
I have had a few dramas with the Cometic head gaskets. Have previously used the Athena one without a drama. Will be reverting back to the Athena gasket & see how that goes.
Despite the drama's I've had the original Athena 290 kit I had fitted was great. Its only the second time around that I've had problems with the head gasket's (3rd one damn it) using the Magnum 290 kit.
ALSO DON'T RE-USE THE GUDGEON PIN (WRIST PIN) CIRCLIPS - make sure you use the ones supplied in the kit.