I've never given that connected/disconnected when running RECOND a thought, and CTEK does not say it should be disconnected, with these newer cars, when smart charging on the overrun/braking, the charging voltage is probably around that level - and remember this, that battery will be acting as a capacitor so the CTEK can throw 15.8V spikes at the battery but the rest of the car should never see these spikes as the battery will quench/sink them.
Unless someone has another idea, certainly I have run the RECOND on my wife's 2015 VW Polo 1.2TSI without anything nasty happeneing, as long as the 2014 Golf R has an EFB then running RECOD will be okay.
It seems that you can also run RECOND on normal AGM batteries, but not, I'd think, GEL batteries, I watched a CTEK tutorial covering all the operating modes of MXS 5 and it was used in AGM RECOND mode - prior to that I considered that RECOND mode was only suitable for wet, flooded batteries and so it would not be possible to select RECOND in AGM mode, not so it seems, so it must be only GEL and not just AGM batteries that can't be RECOND! Though the AGM battery in my sleeping 2011 Audi S4 tests prove/show that it is still in very good condition so probably does not need RECOND run on it.
Edit:- also the maybe light weight desulphating phase that is the normal first stage of charging will also be 15.8V - maybe, I have not checked the CTEK handbook.
Another Edit:- actually the main purpose of the RECOND is to "remix" the probably now stratified battery acid and so return more life to the battery, I don't know why I stuck with desulphating as the main benefit, though that would be why I was questioning the benefit of applying RECOND to an AGM battery that had the acid absorbed into the spun glass sheeting!