The ProSplit R is simply a low-loss isolator, essentially the same as your existing Quick device. So it wouldn't offer any significant advantage.
On my 2014 Cruiser, the standard set-up of alternator and Quick isolator seems to deliver 14.5v or so to the AGM batteries. The Sterling Alternator-to-Battery charger, if set to AGM batteries, would actually reduce the charge voltage!
I don't think you'll get any benefit if you spend £400 on a Sterling A-to-B charger.