V12 tri-turbo...
An all new M Performance built 3.2L I6 (E46 heritage displacement) w/valvetronic w/ two twin-scroll turbos will EASILY put out 415hp (really ~450hp*) with minimal or M5 comparable turbo lag...I don't think any e-assist would be needed....
*M5 specs say 560hp, Edmunds.com dynoed a stock M5 @ 514whp or ~604hp (15%), ~8% understatement. If M3 is quoted @ 415hp x 1.08 = 448hp