might be best to get it on a rolling road and have it looked at, as youre still on MFI theres a chance the system is not supplying enough fuel at higher revs.

the head is restrictive,true,but as it is on forced induction this shouldnt keep the engine from reaching 6500 at least

zetec 2L bore x stroke = 84.8 x 88
lancia delta integrale 16v = 84 x 90