Both cams are replaced from TT

don't get a cam gear, just get 280. you don't need to retard exhaust anymore than a 280's base duraion already gives you.

what can i expect after I install a 280 without a tune, any disadvantages

none 5 degrees of seperation at idle will keep you from stalling. in fact you'll idle perfectly.

i will not recommend going racing or anything cuz you'll be running leaner especially up top with the new flow. we know that evos run rich but just get a tune before you go flogging it. if you stay in closed loop running around town you should be just fine.

I got Zero gains from adding my HKS 280 ex cam on my 9, as a matter of fact, I made less hp than other 9's with out the cam...
Dont waste your $$
Look in the Vishnu forums @ the Socal Tuning trip..