Think I saw one of these the other day. Badged as a 420GSi on a J plate, but unlike all the other 420's i've seen it had fully colour coded bumpers (like the early 216 and 220 GTi), side spats, Roversport alloys and an original grille. In BRG with red bumper inserts.

I'm pretty sure they're called GSi sports, and had the M series engine. Had a twin exit standard looking exhaust too (think most T series R8s were single).

Am i chasing wild geese? :

Edit...
It looked rather similar to this, but in BRG

Difficult to say whether it was a Sport as there were several variants:

420 GSi
420 GSi Sport
420 GSi Executive
420 GSi Sport Turbo

All were badged as 420 GSi - with just detail changes like wheels and interior options distinguishing. Those 5-spoke RoverSport wheels were optional as the Sport and early Sport Turbos had 7-spoke alloys, (later Turbos had 6-spoke alloys), Executives had 'prestige' alloys and I think the standard 420 GSi had steel wheels and plastic trims.

So as you can see its all a bit confusing and as people often swap parts and modify their cars, it is hard to work out exactly which model you are looking at.

Top guess though would be 420 GSi Sport - as you said ;)

I had a car like that once! :

No sport badges just the usual 420 GSi and 16v badges on it, got the spoilers and spat, fully coded bumpers as you say, the one in pic was 100% standard, was bought from an old boy that had had it from new, so unmolested. Came with the 1/2 leather 'lightening' trim, and yes had M series lump, so safe to say, you're not chasing anything