not that I am an expert in this - all this is from the web
but there is a fine line between getting the voltage right to avoid sulphation, but not so high you induce gassing....
sounds to me from the web that 14.7v will definitely get rid of sulphation and fairly rapidly move into gassing phase.
who wants an exploding battery under their bonnet on along run?

not much comment about the fitting of a normal lead / acid battery to a FFRR with systems set for an AGM....

I have noticed that my system also seems to compensate for lower temperatures. On a few nights recently when it has been a little chilly, I have noted my charging system is pumping out 14.9v !!!
In comparison, my mercedes with a normal lead / acid battery is putting out 14.1 v.

I reckon there will be a few people who have installed 110AH batteries, who will get nowhere near the 5 year service life guaranteed. Because the charging voltage of 14.9 may cause gassing which will damage the battery.

In the meantime, I have a 110AH battery sitting in my garage, which I have not yet installed, because i am not able to mak the require changes to the system.
I used to think the comment about the changes being a dealer job was a load of billy rubbish - but I am now not so sure - so haven't made the change.

Hopefully an indy might be able to do it....LR dealer refused. Otherwise I have a 110AH connected to a trickle charger up for sale !

There is another option here.

After fitting my dual battery kit to my L322, I noticed this in the the instructions:

There is also another advantage to this SHARED mode. With continual short drive use of any vehicle, the cranking battery is never properly RECHARGED after starting the motor.

With the DT90 and an Optima battery, because the Optima is a genuine fast charging battery, after a short drive, the Optima will always be in a higher state of charge than the cranking battery.

When you turn your motor off, the two batteries will settle back to their respective states of charge, but because the Optima will always be in a higher state of charge and combined with the fact the DT90 keeps the batteries connect, the Optima slowly discharges back into the cranking battery, progressively raising the cranking battery’s settled voltage.

Over a period of one to three weeks after you install the DT90 kit, your cranking battery will eventually be continually maintained in a fully charged state at all times.

This means your motor will start easier, and as your cranking battery is not working as hard to start your motor, it will have a longer operating life. Current: RR L322 2010
Past: RR L405 2017, RR L322 2012, 2008 & 2004, 2018 D5, 2016 Defender, 2011 D4, a few Disco2s, P38, RR Classic, couple of Disco1's, SIII SWB