Re: 6.5 WSM - vs - 300 WSM for whitetails
You might very well have all the rifle that you need to shoot out to 600-650, might be that you just need to get the personal skill and confidence to make that shot. Texas deer are not difficult to kill, although they can be a very difficult target so hitting one properly with a .27 to .30 cal bullet is the real challenge. Not trying to demean your personal shooting ability, but most guys who want to get out past 400 can use some marksmanship instruction and the more practice the better.
I have found that many factory rifles will or can be made to shoot lethal-sized groups out to 5 or 600 or even significantly farther. You may already have a rifle that will do the job, or that can be worked over a bit to get the accuracy required. If you are shooting sub minute of angle (-1") at one hundred your rifle is probably up to the job.
If your rifles are custom built they probably have a lot of accurizing done already, otherwise you could look at a bedding job, maybe recrowning, trigger job - all good things to do to any factory rifle.
If not you can go two routes, have a custom built which would probably involve around $1500 and up (depends if you supply an action or want something like a Nesika, who is doing the work, goodies used to built with), or buy a rifle like a Remington Sendero in .300 Win. that is intended for those types of shots. There are also some "semi-custom" rifles such as the H-S Pro-2000, or Brown that would be good.
Big factors becomes optics and personal skill. Bottom line easiest way to get the confidence and skill required to kill at that distance would be to contact someone like Bobby Whittington at Badlands Tactical Training near Wichita Falls TX. They do the job for teaching how to hit out to your required distance and farther.
Spend money on ammo and shoot as much as you can. You can spend a lot of money on a supergun, but 600-650 is really not that difficult to shoot to. I would get the best scope I could afford and a bunch of ammo. Then shoot regularly keeping good notes on wind, drops etc. The reason that a lot of guys on this site talk the talk, and walk the walk is because they are backed by good solid records and data on very good rifles and loads.
Your objective is very doable and you can have a lot of fun getting that confidence and skill. Get a partner who is also interested in getting there - then start shooting paper and steel targets. What you want to do requires wind reading skill - that is going to be the challenge to acquire but it is very doable. Who knows, maybe you will want to shoot even farther someday.
I increased my practice distance out to 700-1000 just so that I can have confidence for the exact shot you are talking about. There are a lot of fellows here who can make kills at double or triple that distance. More power to them. I am with you in wanting to shoot confidently from 500-700 yards, that is the level of challenge I want in my hunting at this point in time.
Good luck, sorry to ramble.