What adventure950 says is correct and the measurements ring true with my experience..
With the standard rear end geometry, a standard(ish) shock with 90mm stroke
and a 100mmm swing arm extension resulted in a 60mm increase in travel or 230mm total measured at the rear wheel. (no bump stop)
Increasing the drive shaft angle particularly the down angle (range of arc) will result in more travel.
A longer stroke shock will allow taking advantage of the longer travel and clearnace problems do arise
There are slightly different approaches to the solution of clearnace and shock length while still retaining
a cush drive shaft rather than going to the solid shaft prefered by HPN.
Hogging out extra metal from where the U- Joint / shaft passes through the curve in the swing arm is a help.
You can really go to town with the die grinder and remove everything you can get to.
This solves the problem with the U-Joint grinding.
The next point of grinding is the cush drive assembly.
A beveled edge on the inboard end of the cush drive spring helps clearance at the rear of the tube.
Preloading the spring with one or two 42mm hardened washers again on the inboard end of the cush
(GS front end has a couple) moves the fouling point and carefull alignment of the swing arm position in the
frame (left / right) lets you find a sweet spot.
The way the swing arm is extended will have an influence on how much travel can be achieved.
Extending straight back will produce less max travel.
Extending back and to the left along the existing line of the swing arm will give more clearance for the cush drive.
Coming back and to the left allows you to run a wider rear tyre (140) but increases the rear wheel offset.
Dave Kellet the frame chiropractor tweaked the offset for me so the wheels align.
How he does it is his secret.
At this point you are chasing the last (practical) 5-10mm of travel.
Maximum shaft angle can be determined by experiment you will feel the point where it grinds and whatever grinds
will leave a witness mark visible on disassembly - modify, assemble, test, disassemble inspect, modify recursively.
The lower shock mount can moved forward approx 20mm to achive max travel on a 90mm stroke shock.
Removal of part of the top lip of the rear footpeg mounting plate is required to accomodate the spring.
This change in geometry maximised the down angle and limits the up angle. (shock bottoms)
Raising the subframe was no longer required.
The angle formed between the swing arm axis and the shock body is now closer to 90 degrees at
full compression resulting in a bit more progression to a stiffer effective spring rate on the big jumps.
I'm back to a std length swing arm with 210 mm travel.
The 100mm extnsion resulted in 270 mm.
I have 80-100,000 Kms of hard use on a driveshaft running at this (extreme?) angle.
No problems with wear in the U-Joint, bell coupling or the cush drive so far.
I'm using a 680 lbs/inch spring.
Std was 390 lbs/inch.
I'm 250lbs + riding gear
For me the whole rationale of jacking up the rear was to level the ride hight with a replacement front end.
I'm finding I use only 225-230 mm travel up front of the available 270mm.
I do use all of the available rear travel and would like to have a little more in reserve for those landings when I don't want to lead with the front.
With the 270/270 suspension setup to use all the available travel the changes in geometry could be extreme requiring a very smooth riding style to control the beast especially at speed.
The point of the suspension upgrade was to allow me to chuck the bike around.
I'm thinking a 250/250 setup on a 75mm extended swing arm,a slightly steeper front end (not so long forks) with a triple clamp offset in the 28-32 mm range would produce a great all round package,
I hope these results and observations help you achieve your own solution that matches your needs and capabilities.
As they say in the infomercial - results may vary.
My bike is slow but the earth is patient.