250 bhp in a seven style car is alot unless you are a very competant driver, its easy to get caught up in the big bhp conversations but its really not required to have a fast roadster as they are so light.
I have had a few bmws including an m3. If you think a 3 series with 170bhp is fun in a car that weighs the best part of 1500kg, then 150bhp should be more than enough to keep you happy in a seven.
Spuds car running 2lt zetec/bike carbs is easily as quick as an m3 upto about 80mph and would run rings around it on the twisties
I had a big power 300zx and was always worried when coming out of corners that it was trying to kill me
the back end would go at 80mph in a straight line if you provoked it and thats not fun. If my roadster behaved the same i wouldnt be happy about it.
But each to their own and good luck