I field tested a few 16ga RST 1 oz copper 6 loads on Iowa pheasants last week. Dropped 3 birds with 3 shots. 2 came down dead. I didn't center the 3rd as well, and he came down in some really heavy cover. The dog didn't mark him, and the cover was far easier for a rooster with 2 good legs to navigate than for either me or the dog. Sorry to say I lost that one, but I don't fault either the load or the dog. 1200 fps. I'd go with those in a light gun like yours.
I don't see any reason for a lot of extra velocity. But with British shells, the velocity they list can be misleading. We measure velocity at 3 feet from the muzzle. The Brits use a couple methods, neither of them equivalent to what we do. One of those methods is TRUE muzzle velocity--measured right at the muzzle. 1350 fps at the muzzle works out to about 1250 fps at our standard 3 foot measuring point. So they might not be all that much faster than the RST's. But the RST's are not all that pricey ($11/box for standard lead, $15.50 for copper), and they'll sell you a mixed flat with a variety of shot sizes and payloads.
