It comes down to figuring out what your realistic expectations of the rifle are.
The thing is, while you certainly can build a 223 that will shoot well out to 800+, most factory rifles are not designed for it. You’re pushing the ability of the cartridge at that point. A 1:12 twist rifle isn’t going to cut it, and a 20" 1:9 is a gamble regarding stability of heavier bullets (70+ grain). Whereas, it’s pretty easy to buy a factory 308 that will shoot past 800 without any special needs.
My advice would actually be to bypass the 700, since the SPS Tactical is the only heavy barrel 223 that I see that’s even a 1:9 twist and the stock is a freaking joke. I just bought an AAC-SD (same as an SPS Tactical with a threaded barrel) and I’m not even going to shoot it until I restock it, because the stock will flex and touch the barrel when I’m shooting off a bipod, guaranteed. I knew that when I bought it but just FYI, the stock is known to be a total piece of shit.
Nothing against the 700, I own several myself, but if you want a 223 that you can shoot at extended range without putting extra work into it, there just aren’t any factory 700s that fit the bill.
I would strongly suggest buying a Tikka T3 Varmint, 223 with a 1:8 twist, 24" barrel instead. The stock on the T3 is worlds better than on the SPS Tactical (I own both guns, so I can compare them side by side), and the 1:8 twist will allow you to use heavier bullets, up to the 82gr Berger non-VLD probably, with no problems. It’s even a bit less expensive than the 700 SPS Tactical. An 82 grain Berger BT Match at 2700 fps won’t give up much to a 175 grain Sierra Matchking 308 at 2600 fps out to 1000 yards.
Another option is a CZ 527 Varmint; it’s a 24" barrel, 1:9 twist which is not ideal, but at least the stock is decent (though it looks goofy).
The Savage 10 Precision Carbine and 10 FCP-K are options, but again, they’re 1:9 twist. Also, my Savage 10 FP-SR in 308 is nowhere near the quality of my Tikka or any of my 700s. The accustock is supposed to be pretty good, and they have nice triggers.
The Ruger M77 Tactical is, again, 1:9 twist. I’ve never been all that impressed with the Ruger M77 triggers, either. Looks like it has the same shitty Hogue stock as the SPS Tactical. It’s likely the most expensive of the rifles I mentioned.
There are probably other options in the same price range, but of the ones I’ve looked at the Tikka is the clear choice. There is one downside I should mention, which is that it is a 6+1 round magazine fed gun and the magazines are stupid expensive, $70 or so. Buy one spare, keep it in your range bag, and try like hell not to lose the mag that came with the gun.