Sounds perfectly normal to me. The TC will let you spin both wheels in the wet in a straight line, but if it detects one wheel spinning faster than the other then it'll kick in and kill the fun/stop you ending up in a ditch. It's stability control rather than traction control, so it's trying to keep the car stable as opposed to stopping the wheels spinning. If you don't lift off enough to give the car a chance to settle the weight is still over the front wheels giving you less grip at the back, so you need to come off it altogether and let the natural stability give you the balance back.
Mine annoys the hell out of me when pulling out of junctions in the damp so I leave it firmly off unless it's dreadful weather., and even then there's not that much difference as I experienced at Anglesey a couple of months ago in monsoon conditions: I wasn't any slower on track with the electronics turned off.
That said, I'd agree with everyone else here about checking the profile and width of your tyres compared with stock, and it won't hurt to reset the stability control either (there's a guide on here somewhere, check the FAQs).