https://www.youtube.com/watch?v=-Wek0wLzyL0
This vid gives you an idea of what is involved with a 16v conversion. You would be better off upgrading turbo, injectors, cam, intercooler etc etc on the 8v if you are on a budget. As said before, the transmission/ clutch would need upgrading before any power hike as the M46/7 is not much good beyond 250 bhp.