[disclaimer]Thanks dude,im fully no expert on this stuff and can only really point out where i went wrong with my setup,but ill put my thoughts out on yours as it may spark a few ideas/questions along the way[/disclaimer] lol.

The main problem i see with using a large turbo on the M111 is its relatively low rev range(while this can be raised im not sure just how far on stock head parts)
having a smaller window to produce power means you need lots of boost,and you need it fast,which is something large turbos dont appare to do at any great rate!,but it will be interesting to see how the GT35 turns out,though iv never seen the results the twin scroll sounds like the way to go however there seems to be only one turbine housing available(.78)...so your stuck if you want to fine tune with different housings later!...also i would imagine the twin scroll will require tricky work with the wastegate piping.

Yeh i thought long about going with a slightly larger turbo,GT2871/76 was an easy pick but most likely to give surge without the larger exhaust wheel of the 30 series...but there was to many possible problems,the GT30 wouldnt fit for about 5 reasons + i didnt want any less responce and i didnt want to come across any more surge...it just all costs way to much to fix.

Im redoing the cooling lines after i discovered the outgoing pipe from the turbo water jacket was cooked,its rated for 160 degreesC but it would get much hotter than that once the engine stops,so now the lines will be mostly alloy and i will flare the ends to suit the -6 fittings,im just sourcing a decent 1/2" pipe bender at the moment!.

Are you going with stainless because of corrosion?,if its not corrosion than i strongly suggest you go with cast steam pipe/mild steel...alternatively if you must go stainless use the thickest wall possible dont even consider 1.6mm it will crack.
I know top mount looks good....and it may be the only way you can do it based on the turbo dimensions,however...and its big however....mounting up top means you need to negotiate alot of problems,heat gets trapped up there close to the bonnet especially with the fire wall seal just behind,the wastegate can be tricky to plumb,the oil drain needs to dodge the header,the water lines are just a pita and the downpipe is extra long and subject to alot of stress if not adequately supported.

However you make the header flange make sure you have it machined or sanded flat after the header has been welded,as during welding it will warp somewhat regardless of all best attempts to stop it...later you will find this helps stop loose header bolts and makes for much better sealing.

Wastegate size is an interesting topic,you would best do some research on that as its not entirely based on your HP goal but more so what you will be expecting to see go through it with the turbo and pressure you are attempting to run. also the angle at which the waste exists the manifold plays a large role in weather or not it can regulate,i removed a small ridge inside mine helped significantly when i was fault finding my uncontrolled boost problems...however if i could go back i think the 44mm would be a better choice over the 38 with slightly larger plumbing.

Im not sure about boost controllers but Apexi and HKS seem to have it sorted!

Paul