Not really an expert on charging but, as far as I know most alternators have a fairly crude voltage regulator fitted to them (changing ac produced to dc delivered to your battery). Batteries do not like unstable current being used to charge them, they will accept it and charge up but not in the most economical way. These alternator 'boosters' dont increase power in any way, they simply 'smooth' out the power delivery to your battery alot more than a standard alternator regulator.
We have one that was fitted to one of our old berlingo's and we never had any problems with van starting or system running.
Sterling claim that using one of there boosters is like charging your battery with a smart battery charger.
We dont not use leisure batteries in any of our vans, IMO 'normal' van batteries used as a system power supply (good quality one) like the 'charge' from the everyday running of a van more than a leisure battery.
Most SCR are not zero volt drop, heat and resistance builds up across the contacts and break down occurs in time, using a 100 amp relay simply means bigger contacts less heat and a longer life.
Hope this is of some use.