I've got Geissele barrels in 10.3" and 12.5". The 12.5" replaced a 12.5" Criterion Core and groups slightly better than the Core did. I've never shot different loads through either to see how tight they shoot, that's just based on everytime I zeroed or checked zero. That said, I didn't notice any negative balance issue making the swap as the Geissele profile is still nice.
My only 11.5" in the FN/Spike's CHF pencil. Other than the ginormous gas port, it seems to be a fantastic barrel and has a great feel to it, especially with a 9.5" G Mk14 rail. I rebuilt it with a 10.5" Mk16 and FSB on an A2 upper and it still feels light and handy.
A buddy has an 11.5" Super Duty upper and it is very smooth shooting! The 11.5" Geissele would be an awesome. I'll likely pick up a 16" during the black Friday sale
ETA: I don't know that I've ever seen it in stock, but I don't watch it and I'm not subscribed to in stock notifications on it either.
https://blackrivertactical.com/WP/BRT-11-5-OPTIMUM-Hammer-Forged-Chrome-Lined-Barrel-p207705041