I like Dan's suggestion. If at all possible, try before you buy. He also makes a good point about not turning the A/C on and off quickly in order to test the system.
Honda's have an economy switch and the A/C might not be able to spool up quick enough during startup loads. You might need to run the generator on normal mode.
My sense is that doubling the running load in order to estimate the starting load is one way to guarantee having enough power but that the actual start up load will be somewhat less. I also believe that doubling the running load accounts for the 'power factor' loss associated with using a modified sine wave generator. The Honda 1000 & 2000 (and the smaller Yamahas) generate (almost) pure sine wave current and you may be able to squeak by with the 2000 instead of the recommended 2.5kw.
'Power factor' is an electrical
term that describes how motors relate to the alternating current driving them.
I agree: If you're going to spend the money, test it out first.