Sorry to necro thread but this was the only info I could find on the net about a budget shock upgrade for my CB300F and CBR300R, and I just wanted to add some more info for any new interest.
After reading this thread a few dozen times I finally got my hands on an inexpensive R6 08-16 shock ($150AUD) and had a go at Installing it on my development/track/idiot CB (Front CBR300 forks and YSS fork kit) before throwing one at my partners CBR.
I opted for the frame cut on mine as I've been combatting some stability issues which appeared to be caused by rake angle with the rear preload cranked to combat hitting bumpstop and the front sag set to match.
I made up some shims for the lower 10mm bolt which I inserted into the R6 shock. I used a similar 12mm bolt and drilled a 10mm hole through it on a lathe and cut two 4mm slices.
OD 12mm, ID 10mm, Length 4mm.
I toyed with making my own bearing race but couldn't find someone to make one hard enough.
OD 17mm, ID12mm, Length 32mm.
I was having some trouble with clearance on the triangular link that was binding the suspension, So a flapper disk on a grinder and it no longer binds.
I then bent the ABS back as far as I could with a long bar for mechanical persuasion.
Night and day difference in the handling of the bike, can't recommend it enough. I threw my partner on after riding their own CBR300 and they were astonished as how much lighter/agile the bike felt with the same level of stability. (Identical tyres, brakes, pressures etc etc).
Some numbers for those who care.
CB Shock: Extended 547mm, Static 540mm, Sag 520mm. (preload wound all but one click up trying to combat hitting bumpstop)
R6 Shock: Extended 535mm, Static 525mm, Sag 502mm.
I'm not too concerned about the cut to the frame as its a multi layered section of pressed steel, Although being that I love my partner, I will probably make a smaller and cleaner cut on their CBR300 and have a friend TIG up the edge to add back any lost strength.
As an addition I set the R6 shock to the factory settings and so far so good. I will be getting a friends help for rebound etc at the next track day.
Here are the relevant pages from the owners manual of a 2012 R6.
To finalize, chicken strips are completely gone now, corner exit isn't so damned sketchy any more and my back loves me.