I've built myself several rifles on M700 actions, and most of them will shoot about as well as any of the four built around BAT 3L & 3LL actions, one Nesika K, and two Pierce TGs in Elesio R5 & RTS tubeguns. I've only built one 700 that was a disappointment; its custom bbl was chambered for 6.5x47, and was undoubtedly the pickiest, most finicky rifle I've yet owned. I do agree that using a PTG bolt seems to help insure accuracy, and I prefer the mini M16 extractor to the stamped one in OEM 700 bolts.
I was comparing my current favorite M700 in 260 Imp 30* to the Pierce TG in the RTS last week, and there's no question that the Pierce's bolt runs a whole lot slicker than the PTG custom bolt in the 700. Don't know how much that's worth to anyone that's not shooting rapid fire strings in NRA XC HP matches, but regardless, it's nice.
In my view, unless you're doing your own gunsmithing on truing a 700 (or other factory action), then there's little question that you're ahead to go with a custom action. If I'm going to use the factory bolt, I first send it out to Greg Tannel to have the boltface bushed & firing pin reduced to .061", then grind & turn the knob off the handle so I can thread it for a homemade custom knob. If you're paying for that work, you've just dropped almost enough $$$ to pay for a PTG bolt.
Several times over the past couple of years, I've thought seriously about selling most of my rifles built on M700s to finance the purchase of a couple of Stiller or Pierce actions, but haven't followed through on it. Three of my 700s shoot better than I can hold, and two of them already have PTG custom bolts, so what's to be gained? If it comes down to simple pride of ownership, I've done all the work on the rifles in question (aside from bushing the boltface on the one remaining OEM bolt), and am very happy with the way they shoot.
Back to the OP's original question - I've barreled several Stiller Predator actions, and three Surgeon M591s. The Surgeons do seem to be a little slicker; the only thing I don't care for is that their built-in scope rail doesn't extend forward far enough to suit my purposes.