even though you mentioned without forced induction, there's gonna be quite a few posts still saying ESS. lol.
Good luck to you, as wish to know as well.
I think there's headers you can get as well.
lighter wheels will help with acceleration as well.
2011.75 E90 ///M3 6MT | JB | Blk | CF | ZCP | ZCV | ZCW | ZP2 | 403 | 6NR | 752 [Sold T.T]
'95 Integra GS-R [Stolen-RIP] | '05 Legacy GT Stg 2[Traded] | '08 STi Stg 2+[Sold]