My local friend set his up with ESS twin scroll supercharger about 370 hp street, perhaps 390 on high octane. Is slightly faster than S54 stock e46M but I would prefer it with an Escargot.
E60 and E46 M54b30 has been successfully turbo to about 400-450 ps +/-. some single some twin turbo setups-
The modification must start from cooling system as the M54b30 has too many plastic items!!!
If healthy, the Longmotor is very stout (Mahle pistons and forged rods and forged crank) and can withstand as stock a GT30r without issues- M54B30 has 84mm bore pistons is happier on boost than 86 or 86.4 or 87mm bore.