Yeah NA engines require a lot of mods for smaller gains.
On my old E92 M3 I added 55 whp (most stock M3's dyno at 345 whp on a wheel hub dyno) which got
Me to a tad over 400 whp by doing header back catless exhaust, pulley and ECU tune mated to catless system.
Turbos are fun but once you've gotten used to a fast responding engine like the M3's I just can't stand any form of lag anymore when I drive a forced induction car.
2012 E92-M3 Space Grey on Fox Red, DCT
2011 B8 S4 S-Tronic , Black on magma red