Gunslinger,
The warranty issue is completely up to your dealer that will do your service. They are the ones that will make a stink or not if the rad is removed. If the service manager there says that they will not honor your warranty. If you are nervous about it, keep the stock parts and put them back in if you have an engine problem.
The "cons" would be on icy trails without scratchers or dry trails all together (which would ruin the hyfax before the engine had problems).
Have a look at the diagram and decide for yourself... notice that the coolant does not flow thru the radiator and then the rear cooler assembly. It flows thru both and will take the path of least resistance.
Two reasons to remove the radiator on 900's (and 755's)
1) The sled will run cooler without it if you have scratchers.
Scratchers are mandatory on these sleds, IMO
2) The radiator weakens the front end plastic.
#1...
If you remove the radiator, do not just put a union between the hoses... that will actually cause the temps to rise... Remove the "T" (can be a beotch if you have big hands) and put the union in there. This removes the dead-loop which robs the rear cooler, running board coolers and front heat exchanger of much needed flow.... plus that hose and the fluid in it weigh probably 3+ lbs.
#2
If you think about it, the "radiator" depends on airflow THROUGH it that means that the nose-grille and the area behind the the radiator must be free of snow... When you use the SNOWmobile in the SNOW... the grille and the area behind the rad pack with snow...That turns the "radiator" into a "heater" inside the nose of the sled...adding heat to the under hood area . Remember this is a free-air design, no fan or ducting force air thru this snow-packed grille. At that point the radiator is more like a heater in the "box" formed by the nose pan and "shelf" above it.
PLUS
This "heated box" in the nose makes the plastic soft (really soft) so that when you yank on the already weak design of the RMK nose assy by the bumper, things tear off!
The radiator design was, IMO, one of the bigger blunders on Polaris' part, taking into account the plastic construction and lack of airflow in real world use, ... that they remedied in 2007 and 2008.
Think of the 2008 800 Dragon 163" RMK.... This is probably the lightest, Highest HP stock sled that Polaris has made with one of the highest loads (163" track)... That power and load make a lot of heat... not only does the dragon 163 run in an acceptable temp range in most conditions, it does so without a radiator or running board edge cooler (which your 600 and 700/900's have) AND on the 163's it does it, STOCK, without a front tunnel/bulkhead cooler.
Power to the people.
See any snow on this TRAIL machine grille (much less a mountain machine).
How 'bout behind the radiator??