I have (purchased) and installed the LevelD 767 and the Feelthere 737PIC in their FSX makeovers.
Last night as a bit of a test I noted that these aircraft run at up to 10fps slower, and jerkier, than the default 737 and CRJ. Sure the defaults have no FMC or equivalents, but the performance hit of the clunky add-ons is pretty substantial.
To make good comparisons it is important to keep all other parameters constant. But with these add-ons I have to pull back autogen and complexity sliders to get acceptable airport performance.
I hope the made for FSX add-ons like the PMDG 747 and NGX and those to follow from other makers will truly take advantage of FSX features and go for smooth. These made-over FS9 aircraft literally force us back to less complex models or the defaults to avoid a slide show of an experience.
Sure I can pull the sliders right back. But I remain offended by the fact that there is scenery hidden from me which I can't see, and for which I paid.....I resent that.
And I have a pretty powerful rig by most standards.