I think that you will continue to get benefit in terms of velocity as you increase barrel length, even beyond 26". The question is, "At what cost?" You see, you get diminishing returns all along the way, from the legal minimum barrel length all the way to ridiculously long lengths.
With a 140 gr spitzer bullet seated at normal depth, the .260 will have an expansion ratio of about 8.1 in a 26" barrel. Regarding expansion ratios, a reasonable rule of thumb is that you can get solid gains in ballistic performance as you increase the expansion ratio up to about 8.0, and you should try to do so if you don't have some significant restraint on barrel length.
This is about where you are with a 26" barrel in .260 Rem. You can still get somewhat smaller additional ballistic benefit as you increase the ratio, by increasing barrel length, up to a ratio of about 11.0, if you are not bothered by a rather long barrel. That barrel length in a .260 Rem would be 36". That's pretty darn long. It's about the maximum that you can buy from a barrel maker without an awful lot of fuss. Then you have considerations like, "Who makes a gun case that will fit this?"
Increasing barrel lengths beyond an expansion ratio of 11.0 yield progressively smaller ballistic benefits at the cost of increasing impracticality.
So I would say that if you hunt in wide open country and are not bothered by packing a longer barrel, fine, go for it, all the way up to 30" barrel length. You can benefit from the extra velocity there. If you hunt country that is the least bit forested or brushy, keep to a maximum of 26". If you hunt country that is very forested or brushy, a barrel of 18" to 22" may suit you better, and you might consider changing caliber upward to .308 or .358 in order to maximize bullet energy and ballistic performance from the shorter barrel.
Check this site and play with the numbers;
Powley Computer
Best regards and good hunting.
