Micron system is way lighter than a modified (replaced link pipe) stock system and can be modified in the same way, so preferable to an OE exhaust, beet/mhp is best though.. but £££££££
scorpion systems however are a bit poo, and when i fitted one to an L that had been setup with a micron system it lost power and felt strangled. Sounded good though.
L airbox is totally different to the H and the flow characteristics are different as-well as the total flow.. so required jetting differs (also H and L cams differ)
L KnN is not really any better than clean OE (and needs snorkel removing or it flows worse!!)
full micron (race) + KnN (snorkel removed) + #105 keihin jets without air-corrector's blocked is about right for a stock engine.. with blocked correctors, probably #102
this is the setup from my old L4. (note this was with dynojet needles fitted, and i cant remember what clip position was used, but that only really affects midrange)
my H1 has #102 keihin main jets, OE air-filter, skimmed head, skimmed block, no base gasket, advanced ignition, full micron system, the jetting is lean at the top, but its already faster
dynojet jets flow more than keihin so a #102 keihin is a #100 dynojet .. ish ..
as a complete baseline, my L4 was dynoed when completely stock and was fitted with 2 x #90 and 2 x #88 dynojet jets (inner two cylinders get bigger jets)