Honestly, if it shoots well with that ES/SD just shoot it. I can understand chasing chronographs. We've all been there. But if you're shooting .5 moa groups at 3000fps with the 140 the animals nor the target will know any different.
R26 is going to be hard to beat in the velocity department if you're at max. If not, try working up and see what your top end is. N570 might be faster, but it might not. It might also not be as accurate. I can promise you, it will have more carbon fouling.
If the barrel has 200+rds you're probably about topped out on MV. If not, it may speed up a bit.