I get by with 2 now.
Red n blue will do.
Used to try a lot of things
Until I got a clue.

(Sorry, sometimes I jus' gotta.)

But it's a bit less simple in practice.
For instance, I've found a strain that does poorly in veg. with 660nm red, but seems fine with 625nm?!

It also appears that R:B ratio is a player, strain-wise.
Can't tell you much about flowering stage though.
Doing all my flowering outdoors now.
Not that the leds didn't cut it.
They flowered impressively, but the sun is free here, and is almost 12:12 year round.
I jus' filter out the UV and watch 'em frost up with trichs.
Now I don't need to keep two rooms and chase light leaks etc.
(Way to make a lazy guy happy)

And, yes, 10 - 15nm margin is no problem.
Go for efficacy/wavelength.
(The plants grade on a curve)

"I'd really like to hear if anyone has done more specific experiments on these questions. Should do them myself and stop complaining I suppose."

Gobs of it, ad nauseum.
Google "Martian method" It was a great ride with no real destination. IMO
Aloha,
Weeze