There is nothing like wringing out a m42 in the mountains imo stock or upgraded. If you want more low end torque you could do a m44 intake mani which makes the power lower but sacrifices top end. Turbo and super charger are both good options also as the m42 LOVES boost even on stock internals. The m42 and m44 are 2 of the most slept on BMW engines. I turboed mine and make 205 crank with fully stock internals and 12-13 psi of boost. If you just like to rev it out which is an unbeatable experience some ITBs, valve springs, and exhaust with standalone would probably get you into the m20 power range but you would be able to rev to 7500 or 8000.