The stock shock is 320mm eye to eye, the GSXR600/750 shocks are 315mm(measured on my CNC with a Renishaw probe). So you are -5mm on the shock length.
That makes a larger difference at the swing arm due to the linkage involved. I think it was about 10mm.
Flipping the stock upper mount is either +14 or 15mm(I don't have my notes in front of me).
A good friend that owns a suspension company said he starts with about 330mm shock length when he is setting up a N400. Most of the bikes he works with are race/track bikes and he is setting K-tech or Ohlins shocks to that length as a starting point.
Flipping the stock mount and going to the GSXR shock is the equivalent of a 329-330mm adjustable length shock chassis/geometry wise.
If you just swap the GSXR shock in without flipping the mount, I would recommend raising the forks in the triple clamps(dropping the front lower) to keep the bike from wanting to run wide.
My wife's bike has a 2020 GSXR750(lighter spring than the 600) rear, but the upper mount isn't flipped. The forks are set 35mm above the top triple clamp(not above the handlebar clamps).
I would not recommend dropping the front any further than that, as you could bottom the forks into the lower triple clamp!
My 400 has a stock flipped upper shock mount, 2014 GSXR600 shock, fork tubes set 28mm above the top triple clamp, and I run the taller 140/70 rear tire. It goes where I point it--- on the brakes, mid-corner, on the gas, it goes where I point it.
I have cartridge kits up front that are sprung for my weight, but the shock upgrade(along with raising the rear) was a much better bang for the buck upgrade than the $650 I spent on cartridges(Andreani) and Ohlins oil.
The damping rates on the GSXR shock are on the stiff side for me on the bumpy roads we have around here, but it works really well on the track. I drilled a second hole in my linkage(like @BMHS
) that I use on the street.
I would recommend you read this if you haven't already:
Shock Swap w/cur gen GSX-R 600/750