when thinking about FMIC's remember to put just as much effort into the pipework as you do to core design,
We spent alot of money on our pipework and alot of designs and used 'Mandrel bends' (no more than 90degree) and minimiseds the number of joins (alot of people use cut/weld bends to save on cost) our system only has 5 joins from turbo-inlet and 4 are unavoidable as they are for the trubo, each side of fmic and inlet and we only have 5 welds in the whole of the pipework. and considering the amount of the pipwork, you will not find many others that compare!
Also consider pipework bore size, the stock system uses wierd shapes/sizes all differeing, so if you use a system that retains some of stock pipework u will retain these different bore sizes, ours was a consistent 2.5" from turbo-inlet which is one of the larger to get but not too large so that it has detromental affect.
But dont be fooled if making your own as quality costs £££ and if you were like us, chasing that extra bit of quality/performance costs £££££ and if you only make one there is a premium compared to batch buying
then dont forget end tank design, we changed ours 6 times till we got the mix right of space vs flow vs capacity vs dwell vs pressure loss vs cooling etc etc
plus........ our cores went on everything from 150bhp-400+bhp so if you plan on making your own and pushing your car forward, consider this also