Wayne,
I've had my BPU car for nearly 3 years now. I have always looked at the possibility of going single. I have also gone for a ride on some of the single turbo cars on here (e.g. Terry S's old motor and TRD3000GT's).
Me personal advice to you would be to enjoy and get used to your BPU car first as you've bought it fairy recently. I think I have just about mastered driving my car - knowing the limits in the wet e.t.c and that's taken me nearly 40,000 miles of driving! I'm now confident whenever the rear end fishtails and controlling it, it doesn't catch me by suprise if the rear wheels lose traction.
In my opinion most of the single supras are quicker than BPU but not by a great deal. That's why I decided to keep the car BPU - It does the job and does it well.
Cheers,
Ro