intake don't want to be polished as slight rough surface texture helps fuel air mix . don't want remove to much material either really, exhaust side is where gains can be had easier . Good blueprint of manifolds is a must. Porting is an art form when done real well & needs good experience & flow bench .
I would not touch chinese turbos, seen plenty of the junk in thailand :-D these probably better than some but still under perform & you got fook all warranty plus you looking at potential £100plus import tax bill.
only thing I buy chinese is springrolls, roast duck, prawn crackers & cheap phone repair parts lol .
I'm a big fan of GTX28 & 29 series turbos, if aiming around 400hp you don't need go up to 30 series, GTX28 series can do 500hp easily but real trick spec is indeed big money.
I would hunt around and see what you can get new or used or upgraded from what you already got .
Other option for budget conscious is picking up an evo turbo used as these can be had cheap and build a custom steel manifold from buttweld fittings (steam-pipe) .