Harley wrote:
7 port heads use standard mini exhaust.
8 port head needs a custom extractors with 4 down pipes instead of three.
Mainly a cost saving thing.
Also 2 & 3 fire at different times so they can share the same exhaust port without affecting the flow. An 8 port could in theory flow better but it won't be noticable.
There are a few options for 5 speed gearboxs.
-Road boxes tend to have a "weak" 5th gear and it's not recommended to put big power through it.
-Race (straight cut) boxes are stronger in 5th but it's not like an overdrive so it's not worth it for a road car, plus sc gears are noisy again not ideal for a road car.
The way I see head conversions, 7 port heads are a great "standard" swap. They will bolt straight on and give great flow rate improvments, I've heard 40% power increase. However some aren't as good as other brands and most of them can benfit (some a lot) from a port and polish before use. Also EFI may not be an easy install depending on the brand of head.
The BMW heads are more work to install and probably cost more overall but BMW spent a LOT of money and research getting them to flow great so they don't need porting (but can also be improved if you really really want that bit extra). Out of the box they will make more power than a 7 port, the power will be a smoother curve and they have better cams that can be adjusted instead of the mini cam which is pick one and live with it. The come std with EFI and IMO they are eaiser to swap and modify inlet manifold designs weather you want a single TB or quads.
Both are alloy so are lighter than a 5 port head, both can struggle with space for carbies/TB and air filters. They both have pros and cons and ones weak spot is the other strong spot (depending on what you want).
If you want to really go EXTREAM A SERIES I'd say go 1360 too 1400, turbo, EFI (maybe even water/methanol injection aswell), ECU controlled timing, BMW 16 valve head, 4 speed "std" box, LSD.
Or you could go for a 7 port with a huge turbo with huge boost levels and see if it will survive. Depends on what you class as EXTREME.