I doubt the original driver even hits an amp - Lumileds LEDs rarely got driven at over 700mA. A driver swap as well would be a pain, especially if you want to keep CR2 compatibility - mine doesn't like RCR2s. I have a couple of bare R5s which it might be possible to fit on an old 20mm star. Most lights of that generation used 20mm stars.
With a bit of fiddling and reflector cutting it might be possible to get both of them into it which would be a fun light with a driver swap. Rather hot runnning though.
The 2 lumiled flashlights I modded were about .7 or .8A stock. Because of the reduced resistance of the xpg, they both now push over 2A and they are both completely different designs with stock drivers. Obviously not truly current regulated but most cheap drivers arent. Also, the stock MRV pushed 1.05A with the xre and now pushes 2.35A with the same driver on xpg. I think a 16mm star would work well in your 606, be pushed HARD and allow you to center it perfectly where the 20mm came out. You could remove the 606 reflector and place it temporarily over one of your other xpg powered lamps to test the beam pattern before hand. Just hold it in place with your hand, turn on the light and you'll know very quickly what to expect, or what shimming/trimming may be required. I suspect you would be very happy with this mod if all goes as Id hope. Of course these are only suggestions. IMO, The 606 sure is a fine light to not have in operation and appreciated.
I also had similar experiences with my SS C3 xpg mod here - from xrc, no driver swap and more amperage being pushed through the stock driver. Im very happy with all my xpg mods and they were so easy to accomplish. Without the xpg's they'd still be sitting in in a drawer or gifted to someone who wouldnt use them either. Another benefit is that most of my hosts have a superior build quality when compared to the same trashy models they are trying to sell today.
That's useful to know - may just dig out the soldering iron and play. But I'd better finish the balance harness and holders for 6 18650. More to discharge them for storage since it then becomes easy to charge them up 6 at a time.