The engine itself would probably be pretty comparable as far as weight goes unless they made the block out of magnesium (which would mean all the piggybacks would be potentially more risky). Just because it has 2 less cylinders doesnt mean its going to weigh a lot less because you have to factor in the weight of the turbos, the piping, the intercooler etc.
1995 M3 Coupe - HPDE/Track Prepped - For Sale
1997 M3 Coupe - BMWCCA I-Prepared Race Car
2011 335d - Das Uber Commuter (35.1MPG Avg)