'99 SJ FSRxc w/ MRP (3"x3.5") & '03 SJ FSR w/ BETD (5"x5")
Just found out about MTBR's "Manufacturer Forum" - I didn't know about this until last night (duh!)
(And look whom I find here...

Hi, S-Works, good to see you here!)
Anyway, here's my FSRxc story...
I used to have a ''99 StumpJumper FSRxc w/ MRP (3"/Marz Atom 80 x 3.5"/MRP) - pic below. With this setup, I'd gone for some up-to-3-footers air's/drops. (I'm only 150lbs + 15lbs/gear.) Boy, I was ever-so envious of S-Works nicely putting a BETD link on his '99 FSRxc instead. Anyway, with this bike, first I got a crack line near the infamous "pinky hole" on chainstay (just near the BB / main pivot), like everybody else did. Speciazlied sent me new chainstay, which was exactly the same as the stock one I cracked (instead of the newer silver replacement chainstay piece with smaller "pinky holes" that S-Works received as replacement). Needless to say, then I cracked it in the same spot again. This time, they sent me a yet even newer & beefier black replacement chainstay piece with smaller "pinky holes". No, I didn't crack this chainstay piece, but then I got a crack line near the seat-tower. By then, the MRP link had crack lines all over the place, too, so I guess it was a matter of time. (The folks at MRP couldn't do anything for my cracked linkage, just FYI.)
Thankfully, this time Specialized gave me a whole new '03 StumpJumper FSR frame as a replacement. (I still think it's basically a "FSRxc", but they called it just "FSR" in '03, until they had 2 diffrent "FSR" & "FSRxc" in '04.) They're
not kidding when they say "life-time warranty" on their frames. Anyway, I made sure that I went with BETD this time (especially after all that envy), and I haven't regretted it since. (The folks at BETD are helpful & responsive via email.) So, I'm now set up with 5"x5", thanks to a TALAS fork & a BETD linkage - pic below (30.1lbs). I really enjoy this setup, and I agree with S-Works that 5"x5" for FSR/FSRxc can be nicely set up as an all-around / trail bike (or whatever one calls it). It surely works for me & my weight & my riding style & the local trails here.
After 750 miles, I haven't cracked anything so far..., yet. But if anything were to crack, I'm pretty sure that it wouldn't be the BETD linkage, but rather somewhere in the rear triangle again. With this setup, I finally started to have some guts for up-to-4-footers air's/drops - pic below. With 5"x5", my bike seems surprisingly confident/comfortable/capable (thanks to TALAS & BETD & 185mm rotors) for a lot more stuff, while I also
try to ride/air/land smoothly on my part (well, as much I can, you know). However - this - I feel that I might be pushing it a bit much now. I probably won't "get away with it" for long, even for my light 150lbs weight. So, in that sense, I agree with DJBiker - now I should probably start looking for something else (ie. Heckler, Saber, Preston-FR) for my new trail targets (if I want to keep this up & more), as I realize that I'm reaching the "point" of needing to do so.
Anyway, the important thing is this, in my opinion... The "point" of needing to look for a different frame is totally dependent on each rider's weight, riding style, smoothness, local trails, aimed stunts/usages/needs, etc. Everyone progresses year after year, but also towards different directions (ie. some for bigger/FR stuff, while others for faster/XC racing stuff, or whatever else). For some folks on certain trails, a 5"x5"-setup FSRxc can be perfectly competent/sufficient (like it has been for me, going both uphill & downhill), while it may not for other riders/situations. Each person simply has to realize how much one can safely/comfortably get away with a "beefed-up" FSR/FSRxc. Pushing "a bit" is OK - I'm sure the bike can take it. But pushing "too much" is obviously not. And these are all relative & vague terms, so what's "OK" vs "too much" depends uniquely on each rider & his/her needs.
On top of all that, as we all know it, everybody's definition of "trail ride" or "all-mountain" or "aggro-XC" or "do-it-all" (or whatever else) are all perceived differently to begin with.
Anyway, I'm no "guru" here, but I thought I'd share my story & thoughts.
Good luck & have fun!
- PiroChu