Any rear wheel drive will get lairy in the wet on roundabouts if you arent careful on the gas, especially if you have got a couple of hundred horses on tap...
Gently gently is the answer...use small amounts of gas, in higher gears and turn smoothly and avoid erratic movements in steering or acceleration/decceleration...
Get your tracking checked as if the car is toeing in its more likely to flick the rear out, and as mensioned above tyre pressure/tread pattern and shocks can have a bearing...
Having some weight in the back helps, but hard shocks, overinflated tyres etc will tend to make the rear bounce on our poor road surfaces, and any bounce will result in a loss of traction/wheelspin if you have enough horses on tap.
Any gutsy rwd is gonna be "entertaining" to drive in the wet, be it a BMW, caterham or even transit or 7.5tonner!
Oversteer is something that will be unusual to a FWD driver (unless driven hard with lift off oversteer)...but is easy to learn to control it so it doesnt alarm you too much when/if it happens, and you will end up a far better driver for it.
If it breaks out dont do anything erratic or panic, simply turn gently into the slide and gently ease off the throttle and the car will come back into line...
backing off the gas suddenly will throw the weight bias forwards making the rear lighter and more likely to slide further, or could cause the car to snatch back and slide the other way if you have dialled in a bit too much opposite lock.
As for steering into the slide, dont pile on loads of opposite lock...this will only over correct the slide and throw the back end out the other way...Simply keep the front wheels pointing in the direction that the car should be going and use a gentle decrease in throttle to bring the back back in line...Really it doesnt matter which direction the car is pointing in a slide, so long as it is travelling in the right direction, so dont rush to straighten it, gently bring it back in line and you then wont end up fishtailing up the road or spinning it.
Try and find somewhere to have a practise, and you will get used to it real quick...
You think your fronty is bad...you should have tried driving my old mk2 escort...RWD Normally aspirated Cossie lump or 50's giving 250ish bhp at the rear wheels, cut slicks, welded diff and bugger all weight!
