uk turbos are actually smaller then the jspec but handle more boost due to being steal mid to late 500 on stock turbos is quite a ask and they normally have to be put in TTC which makes it laggy as hell , and is not worth the power gain in a auto as the TC stall point is 2000rpm and the turbos in TTC come on around 4000rpm will you have more power yes will your car be faster defiantly not , drag racing a full bpu in sequential auto with around 410bhp was beating mine and other single turbos with 500-600bhp and the single turbos spool faster then the TTC stockers .
I have had my auto stock , bpu , stage 3 hybrids , single turbo 524bhp. and if im honest I liked it best when full bpu for day to day driving as a weekend toy it is now single is good fun but still left standing until spooled up which I don't like too much on the street as your doing speeding ticket speeds by then.
lots of factors to think about its not just power its putting that power to the ground and how that power comes on