Imho, if you are looking for an easy upgrade beyond the 'GSR' twin DCG306 setup, then the Weber 40DCN running east-west over a modified single carb manifold is probably the best option. If you want to go beyond that with dual carbs mounted directly on top then you may get more top end BHP but I think the later engines with outside positioned intake valves and siamised exhaust ports probably limit this.
I did run a Weber 45 DCOE on my GSR at one point with a couple of custom tube elbows that bolted onto the existing carb flanges which did provide increased top end... adding another 5mph to the ultimate top speed. A 40DCOE might have been a more appropriately sized option as we had to run fairly small chokes from memory. Had to feed the throttle in gently at low revs but once it his 4000 it revved out like a rotary... fine on the track, but a bit tiresome on the road.
So my main take out is that even the dual DCG306 under-carburates the old EA63S gives v.good all round performance, although one thing the Weber did is cure the fuel surge in right-handers on the track. The DCN setup would do the same.