In reply to Hardonicus: The old Polar Proof (which was a wash in) used to be recommended by Nikwax for ski wear and insulated garments, I don't think the newer version of it mentions anything other than fleece on the bottle though.
A wash in wont "compromise the insulation", all the insulation does is trap air, if the fibers are water repelant then they will still trap air, they'll just be slightly more resistant to water. The fibers making up the jacket are plastic anyway so wont soak up moisture themselves, but when weaved together they'll hold water. Reproofing them will slightly hinder that effect, although the most advantage will be gained from the water repelency on the outside. The liner being reproofed might slightly reduce the amount of liquid soaked into the jacket if you put it on top of a wet rain coat, however rubbing and pressure are pretty bad enemies of DWR so you might not really notice any difference. It wont reduce the breathability.
You won't go wrong either way, I wouldn't worry too much about it, it's far easier to use the wash in stuff and you get better coverage imo. I'd do that, if I ever got round to washing my belay jacket that is.
P.S. conjecture and ukc discussions of reproofing go hand in hand I'm afraid
In reply to Hardonicus: Polar Proof and Cotton Proof are the same stuff, just different instructions according the Nikwax rep I spoke to and they are basically the strongest concentration of what you get in TX wash in. Tx Spray on is the weakest concentration due to the need to go through the spray nozzle... for which reason I always get the Polar Proof as you need less.
After treatment put the jacket on a radiator which will improve the beading properties.