dirty30

It only changes the timing advance on the WOT timing maps. Expect 8-10hp over stock. 

93 octane may not absolutely be required, especially in winter. I ran 89-91 without issue on a MarkD tune. 

MarkD chip + 19lb Bosch Design III injectors is definitely the way to go for motronic chip tuning if you were trying to get a little more out of it. 

For reasons of packaging, it is extremely difficult to fit a proper intake on an M20 E30. The cruise control, abs motor, coolant overflow (late model), airflow meter, and headlights make the space extremely tight. The Kamotors air intake is pretty decent because it provides heat shielding and it fits, The filter is very cheap. If there was a higher quality replacement filter then I think it would be the best setup. You CAN find the same intake on ebay for less money. People believe "Kamotors" fabricates the kits himself but he is merely a re-seller. Nothing wrong with that, its just the community thinks they can only obtain those intakes from Kamotors.
 
I was able to fit an ITG foam filter (best air filters IMO) after removing the factory AFM and running a megasquirt ecu. Most cone filter kits/adapters for the stock AFM provide little to no transition from circle to square which causes for massive turbulence and a noticable loss of low end power. The stock E30 exhaust system is a wonderful piece in my opinion but the muffler can be changed out for a less restrictive one. 

I dont suggest headers or a full exhaust system unless you plan on extensively modifying the engine.
 
The ROI is pretty poor and the money spent could be much more satisfyingly applied towards tire,wheel, suspension, and brake upgrades

K&N drop-in filter + magnaflow straight through muffler is my vote.