your 20 lb. cap has nothing to do with recovery of coolant, forget that idea. you can go to a 22-24# cap and you might not lose any.
personally, it's hot out, with a 160 deg. thermostat my car will idle all day at 180, with a 180 thermostat it is going to hit 180 and go over that, so you gonna watch it? at what point do you shut things down? mine gets up around 200-210 i get nervous, especially if i'm not running it hard. if it is cold out and you want more temp cover a portion of the radiator, and if anyone asks tell them it is an old nascar trick.
remember, you crack the radiator cap and you lose cooling capacity until it cools again.
put the 160 hi flow thermostat in and get a 22-24 cap and get your overflow set up and forget about it.
http://www.stewartcomponents.net/Mer...ory_Code=Therm
guaranteed!
also, when the thermostat closes there is no flow, so cooling does not improve in the radiator, the radiator requires turbulent flow through the core tubes to cool the fluid, picked that up off the stewart website.