Andy; I've never owned a GRD, but the A16 has a similar zoom range to my old GX200. The 16MP Sony sensor is just amazing, though obviously a GXR with an A16 slotted on is a much bigger camera than the GX200, which performed very well for a 2008 design/sensor and had the shirt pocketability which the GXR doesn't have.
But it does drop into a smallish lowepro bag which I hardly notice the weight of, along with a P10 zoom, or an A12-28. The combination gives much more flexibility than a camera with a small zoom or fixed prime. This suits my eclectic and opportunistic photography style and I'm happy I took the plunge, forcing myself to sell the GX200 as soon as the GXR arrived. I picked up the A16 half price of ebay, and I'm delighted with the results I'm getting.
Additional bonus: The A16 shoots full HD video very well, though the sound isn't fantastic. A port for an external stereo mic setup would be nice. Maybe Ricoh could make one which slots onto the hot shoe and uses the multiport in a future GXR upgrade.