G'day Bula,
Glad to help where I can,
If you are experiencing an increasing mag drop due to increasing richness then I imagine that you must have a very very rich mixture. The simple test is at shut down just pull to idle cut off very slowly whilst watching the tach. Just prior to the engine finally quiting you should see about 25 to 50 RPM increase as the mixture reaches a best power mixture ratio. If you have a greater increase then you are to rich, less and to lean, so just ask engineering to tweak it it's a simple adjustment.
If you have a fuel flow meter a 290hp I0-540 should be indicating a fuel flow of 2.8 to 3.2 GPH at 1000 RPM, (roughly) 4 GPH is to high.
As for lean taxi/runup masking a FCU issue, can't quite fathom that, might need someone else to way in on that one.
A simple FCU check is to see consistantly correct values of fuel flow matching with left and right engine plus or minus a little and a value of at least 140 lb/hr a side at take off and into the intial climb that is for I0-540 E2B5 290hp.
Best place for your numbers is the Engine set up manual. As just because the hp figure might be similar to your engine other issue's are at play like compression ratio and the need for greater richness for detonation suppression for those with higher ratio's.
Anyone thinking of quoting those numbers please check validity first, I'm going off memory here, so just use them for a starting point for curiousity.
If your SOP's say full rich then unfortunatly full rich it is being a maverick in the field isn't prudent.
But that doesn't mean you can't try and educate the policy makers or put your thoughts forward for the merits of change. At least it shouldn't.
Regards
M