I have no experience of this company or its chips, however I'd rather have intimate relations with an orangutan than fit one to my car. Get something from a reputable company such as TME, RICA, HLM, BSR. Fitting an ebay type chip is asking for trouble.
Is an 850 T5 good for 250bhp? Easily. The basic T5 engine is the same in an 850 T5 as it is in my phase 1 V70R and that left the factory with 240bhp as std (auto) and 250bhp in manual cars. I think 250 is pushing it with a 15G turbo - you'd be better off with a 16T or 18T. Add a good exhaust and remap and you should see 260-280 pretty easily.
If you do fit your chip, please let us know how long your engine lasts... yes I'm being serious
HTH