I've had a muffler added where the crossover existed previously. (twin 2 1/2in and twin 2 1/2 out. ) It's still a bit too noisy so I guess the next step is to change the rear mufflers.
I can't be 100% sure but I think the muffler in place of the split has improved performance, especially higer in the rev range. Is it possible that the split was restrictive?
Yes, it is WilliamsF1 car, now in Perth.