I have a stock fitted to my MPR action, the length of pull is very long with the MPR trigger assembly, it's ok for me as I have a long LOP anyway, but others struggled or couldn't even reach it. I can't comment on the standard S4/500 style blade on the S510 US as I haven't shot one. I like it for positionals more than my Gary Cane !
Air Arms are looking into whether it is cost effective to change certain parts of the trigger assembly& poss changes to the trigger gaurd on the MPR to make the LOP better with the US stock though.
I thought I saw someone at Fort with an US, hopefully they will pop up on here.
In life you should not persue goals that are easily achieved, develop an instinct for what can be only just achieved through your greatest efforts.