The big bore kit takes the 800 out to 1000 cc. A similar kit is also marketed as a performance (+ 11%) upgrade to the standard 1000 cc motor. Below is what the kit contains ...
"Description Part No. Previous Quantity Price New Qty
UPGRADE YOUR 750cc OR 800cc (1979 ON) TWIN TO 1000cc. THIS KIT CONTAINS 2 * NICKASIL BARRELS AND 2 * 9.5:1 HIGH COMPRESSION GRAPHITE FORGED LIGHTWEIGHT PISTONS. LIGHTWEIGHT GUDGEON PINS, CIRCLIPS AND PISTON RINGS ARE ALSO INCLUDED. MAXIMUM POWER OUTPUT IS INCREASED BY 26% AS WELL AS A 30% TORQUE INCREASE THROUGHOUT MOST OF THE RPM RANGE. THERE IS NO NEED TO ALTER THE ORIGINAL EXHAUST / HEAD / CARB SETTINGS. KIT INCLUDES TOP END GASKETS (15865* 1) (ALL GASKETS AND OIL SEALS REQUIRED)"
The motors is in both my 1000RS and 800 Cafe racer are so soft and usable it would certainly be a shame to spoil that characteristic in search of a couple of BHP. I'm leaving the "Zen" like RS alone for now as it is already has the twin plug, gas flowed and re squished heads. However I aim to get the Cafe Racer which would have started off at around 50 BHP, up to around 70 BHP with lots of low end torque. With the kit and help of the R90s heads with 38 mm Dellortos, we should be close to that. I'm not sure how to describe it but the effect I'm after is the "long shove" that comes from a low reving, high torque motor in a lightweight chassis.
The Rs engine work that was done, including the up rated coils etc, cost the previous owner around 2K. It works very well and is much more tractable than my old R90s ever was. So Will the twin plug conversion is definitely beneficial and maybe you can get it done for less?

I'll let you know how the kit pans out.
Oh and Dave you are welcome to pop by any time.