With a factory gun their is a chance that you'll get a good one and it'll shoot in the 1/2 moa range , I've even seen some factory guns shoot around the 1/4 moa range in varmint calibers. role the dice and see what comes up.
With a full custom with a custom action and being put together by a good smith , I would expect that you would surely be in the 1/2moa range probably less , your barrel is gonna foul alot less , and you can taylor you gun to what you want like barrel length ,weight ,stock shape and a host of may other things.
You can have a Rem700 built up and it'll be very accurate , even go as far as having an oversized bolt and so on but in the end you have a Rem700 no matter how much money you throw at it and in the end probably not have al well built action as the new Lawton.
But simply from an accuracy standpoint , I think that maybe if you luck up and get a "good" Sendero , have it bedded , trigger worked and make sure it fits you right which is a big accuracy factor ,then you may not have anything to gain by going custom.
My friend sent a Rem 308 VS to Chris Mathews had the barrel cut down to 22" , lapped the lugs bedding the action and it came back shoot 50% better and it'll shoot hand loads in the .3" range regulary and the chamber is kinda slopy , I recomended that he have the barrel set back a quarter inch and have the chamber recut to a match spec but hes happy with it.
But I doubt that many smiths will offer an accuracy guarntee ,in a caliber bigger than 300Win mag, without a brake as few folks have the ability to shoot a hard recoiling rifle accuratly.
But from my experiance , one you have a nice custom gun built the way you want its realy hard to enjoy a factory gun again.