First of all, if you have the barrel handy, you can measure the twist yourself, rather than wait for answers as to what might be the twist. If you have a cleaning rod and fresh brush, run that thru the bore until you just about get to the muzzle. Make a mark at top dead centre on the cleaning rod just where it comes out from the back of the action or rear of your bore guide. Pull the cleaning rod back towards you, making sure it doesn't slip(it needs to freely swivel in the twist) until that mark is again top dead centre. Measure along your cleaning rod from that mark to the back of the action or bore guide, wherever you made your initial. That is your twist rate.
Second. If a bullet is stable at 100-150yds, it's not going to lose stability in the next 100-150 yds. There's probably some other reason.
a 10 twist is the absolute minimum I would use for many of the 100gr projectiles in 6mm/243, and who knows how close to 10 some are using in factory pipes unless you measure it yourself..Better to use faster than 10 if going to the heavy end of the spectrum of 6mm bullets. The reason Remington and the like would use 10 twist is because they are more prone to consider 85-90 ish grain bullets as the heaviest, aren't making rifles for the UK market. I've seen enough difference in group diameters versus twist rates that anyone asking to rebarrel the 6mm for deer would get a recommendation of faster than 10 twist.
JR