It simply comes down to whether or not the SR-15 configuration is what you want. If it is, buy one without a second thought, and be thrilled.
If you want something that is much different, then the value of those features is harder to justify.
A more fair comparison would be what can you do with a DD V5 LW, GSSA, BAD-ASS ambi selector, and B5 SOPMOD/IMOD/STR stock.
The MOD-1 SR-15’s are just amazing feeling rifles, I’m still seriously contemplating a very cost-inefficeint update on my Mod0 to put the URX3.1 rail on it, the handguard is that good. The highest compliment I can pay my DD V5LW unit is that the feel is as good as the SR-15 it lives in the same rifle bag as - but this is a DD carbine with a BC1.0 and a VLTOR A5 on it, that is otherwise a very light stick.
This probably isn’t much help, so I’ll go with the part I am sure about - if you like the KAC feature set on either half of the rifle, GET THE COMPLETE RIFLE. Buying either half of them is not as ideal as you’d think cost wise, and basically it’s hard to buy a better other half of the rifle anyway - with great effort I can cobble together a lower that is comparable in value to me (Noveske Gen2, GSSA, Magpul everything, VLTOR A5 kit), but for more cost; and as gorgeous as the few Colt 6720/URX3.1 uppers I’ve seen are, the cost with those BUIS creeps right back up there.
At this point, the most competitive setup against it would be a 16" Middy (Noveske N4, Centurion CHF/LW, DD CHF, BCM BFH) Barreled unit with a 13.5" NSR rail, get the new MagPul BUIS, and put a hybrid muzzle device on it - you’ll be close on weight, performance, and balance, and a few hundred under on cost. IF you feel you need that sort of upper anyway.
A lot of the features on the KAC are something like a Track Package for a sports car - a few grand for really good brakes looks good on paper, is practically useless on the road, but the first couple times you really run it hard, if you ever do, it’ll be the best money you’ve spent. I think of my KAC stuff the same way, and then usually get upset at myself for not wanting to spend more money on ammunition to really get some quality use out of it.