You maybe right about the dump pipe being to small. Back to back testing on a neo stagea with a hks gtrs, which would be similar size to your hiflow, a bell mouth dump got 7rwkw more top end over the split dump pipe. The tuners believed the difference was caused by the split dump being too small.
Pm AFRO, he has a spare R34 GTT bell mouth dump for sale, or he may lend it to you. He lives at Rochedale