You could quite probably take the latest K-series/N-series or whatever it's called this week, up to 200bhp with the right mods and enough mods. Im not thinking of a simple re-map with more boost as we know that:
1, VVC160 pistons were the basis of the factory K-turbo, and they are not happy much over 180-190bhp long term.
2, The non VVC head is very restrictive on the 1800cc engine, and with the turbo this is going to be even more of an issue breathing wise.
A full turbo back 2.5" de-cat exhaust, panel filter and a bigger I/C would be my first mods to an MG6. Then id look at seeing what potential there is in the std ECU for re-programming. I suspect that it's going to be far less tunable than MEMS3 if at all?
I'd also look at flowing the head, and porting the inlet ports to match a VVC inlet manifold and 52mm throttlebody to the head. There is probably some improvements to be made with porting and port matching the turbo exhaust manifold too.
We have to assume that the old K-series VVC head cannot be bolted onto the new engine, and they don't use this head for the new engine's which is a shame as it would have added about 15-20bhp at the same boost level as std!
If all the above were done to an MG6 engine, then as per the old K-turbo's you could be looking at 200bhp at close to std boost levels and using the std turbo. Putting a bigger turbo on would definately help push past 200bhp if you could tune the ECU, and could accept a big more lag and a slightly higher boost threashold.