I've got a set of Peter Allen (not the singer) crossover 4 into 2 race pipes from the 1970's, they really gave my bike a lift, I took them off when I scored a genuine Yoshi 4 into 1 which wasn't any improvement over the 4 into 2.
The Yoshi's gone now, so I think I'll put the 4 into 2 back on again. Advantages? Equal length primaries, better low end torque, better sound, and I forget the rest, ha ha! Cheers, Terry. ,D