There's no doubt about the cost effectiveness of bow ashigaru - even without upgrades, they cost half the upkeep and are only 28% less effective than bow samurai against katana samurai. But for your main army, you may want to pay extra to get the best you can.

Right now, I'm thinking of going for a mix of bow ashigaru and bow samurai in my mid to late game frontline armies. Say 3 ashis to skirmish in loose formation and take the casualties; and 3 bow samurai behind the lines to add weight to missile duels. Monk archers might well be even better in the latter role (because of the extra range).

To investigate many stats like accuracy upgrades, you'd have to start modding the unit stats. I would not be averse to doing that (e.g. I'd be very curious to compare the smith upgrades for melee, armor vs attack skill), but don't know how at the moment. If anyone with modding skill can explain how or point me in the direction of a "how to..." thread, I'd be grateful.