I think it is possible to objectively rate add on aircraft (up to a degree though). For example I listed a few aircraft and their FPS performance earlier in this post. I got the idea for this from reading a review of an Airbus where the reviewer gave a percentage for frame rates based on the default 737. If you know how well the 737 performs on your system and the addon aircraft gives say 80% frame rates (100% equals the same rates as the default) then you can make a judgement on whether or not this will be acceptable to you.
I think this would be much more helpful to anyone reading a review rather than the more general (and highly fluid and subjective) terms such as "acceptable rates on my system".
Of course this may not be 100% accurate as processor speed versus graphics card speed versus RAM may have an effect on how well things run. I suspect however that aircraft performance in 2D and VC has more to do with processor speed than graphics card (maybe scenery performance is more weighted towards graphics card than CPU speed). I have the ifly 747 which in the 2D view with no panel displayed gives frame rates near the default 747 while with the 2D panel it cuts frame rates to about 50%.
I don't expect publishers to start giving frame rate comparisons (how many extra copies of FSX do you think Microsoft sold because of the unrealistically low minimum specs they give) but it is something I'd like reviewers to try.
Any other thoughts or am I just talking through my hat?