Howdy! Fellow slobbering red-eyed nail-biting GS850G-abusing lean angle addict here.
The only non-anecdotal evidence I have, really... sorry about the mismatched clearance rain gear.
Properly sorted out and tastefully updated, GS shafties handle remarkably well. Suzuki really got the geometry right on these.
Yes, they're heavy (and the 850 is the heaviest of all) and they're not insanely powerful. But a well-running GS850 has one of the more entertaining engines you'll find, with a giggle-inducing rush and howl from 6,000 rpm to redline. The 1000G and 1100G have more power and torque, but I much prefer the 850. And there's an old saying that it's a lot more fun to ride a slow bike fast than the other way around.
Anyway, yes, there are various projects that have grafted a more modern front end onto a shaftie, complete with 17" front wheel. The most common difficulty is that most of the available choices are quite a bit shorter than the original, so the front end is lower and thus cornering clearance is badly compromised. There are ways to deal with this, of course; choosing longer donor forks from naked or dual-sport bikes or crafting fork extensions, etc.
Out back, the limitations are a fair bit more severe, because there's simply not room in the swingarm to go much wider. I can't recall any successful projects along these lines, although there have been many inquiries hereabouts over the years. I do know the VX800 used a 150/70-17 rear, which was technically a little too wide for the wheel, and that the VX800 wheel will mate up to the 850 final drive. However, the 150 tire contacts the GS swingarm.
If you are able to fabricate or modify the swingarm and frame to address this, then you are operating at a level well beyond anything we've seen here and you don't need our input. We do respectfully request lots of pictures...
The handling limits of the GS850G are set by frame flex, not the tires. As Steve noted, there are some excellent choices for very sticky rubber in the appropriate sizes. The Avon RoadRider is well-regarded, and the Pirelli Sport Demon is probably the very stickiest of all, although short-lived. The Michelin Pilot Activ is a relative newcomer and may even dethrone the Pirelli. However, I don't recall any reports yet from out on the ragged edge... we're an, uh, frugal group, and the Michelins are very spendy.
Personally, I've been running the Shinko 230 for quite a while. They're sticky, neutral-handling, and cheap, but short-lived. The GS only gets about 3,000 miles a year these days, and they last about 2,500 3,000 miles under my abuse. They behave flawlessly down to the wear bars, and I like having fresh tires more often.
There have been a few projects that experimented with frame and swingarm bracing on a shaftie with satisfactory results.
I've also wondered whether replacing the stock 37mm forks with the much stiffer 41mm forks from a GS1100GK (and thus the slightly wider wheel and the 110/90-19 front tire) would be worth the weight penalty. There may be other stiffer front ends with sufficient length. Still, my opinion is that out on that ragged edge, it's the frame flex that's far more of an issue than the fork. Tire grip is a distant 14th or so on the list of things holding you back.
And like any bike, the more you spend on suspension and brakes (and spend the time to set things up correctly), the more you'll like it. I'm using Sonic springs up front and Progressive shocks out back, which is kinda the cheapest option.
As far as shocks, sky's the limit; you could install $2K Ohlins, but most of us slap on a set of sub-$300 Hagon or Progressive shocks (there's also YSS, Ikon, and many others). You can also install emulators in the fork, which I haven't done.
I've experimented with a fork brace, but strangely I didn't like it. I think my brain got used to using the information provided by the flex from the forks. But YMMV, of course...
If you want to raise the rear a bit, you're limited by acceptable driveshaft angles. About 1/2" taller seems to work well. I haven't done this.
Stock brakes with braided lines, fresh innards, fresh sticky name-brand pads (I'm using EBC Red) are shockingly good compared to what many are riding around with. Still, the "twinpot" brake upgrade has been reported to have excellent results. It's also important to make sure the front end is up to snuff; the biggest immediate everyday benefit to proper straight-rate fork springs is drastically reduced brake dive.
I actually have understanding of frame flex, suspension, and *gasp* rake and trail and their characteristics. I thought we were in a mods/customization sub forum. If you look at my sig you'll see I have a handful of machines, and these are only the ones I own right now. I have 30 years of riding dirt, street and track under my belt, and I'm not the worst welder! I don't put bikes together with "no", but thanks again for your input. I'm looking for alternatives. Stories of creativity, fabrication and possibility...not "you ain't got the right bike."
