Very slight missing normal driving, poor fuel mileage, lacks full power

Yes, car (Cartier) has dual exhaust. At idle, I can hold my hand on each, against its pressure, and prevent exhaust from coming out a lot easier than other cars, therefore they must be connected somehow after each catalytic converter, I suppose. Someone tested the O2 sensors with fancy diagnostics and fuel trims and were normal, and told me that whatever was causing the slight miss and the extra fuel, was being digested by the catalytic converters.
Will still try to do the test you suggested. Someone to push the brake and gas pedals in gear as in a load condition and I to compare pressures at each exhaust pipe and will be using my nose for smelling differences. Thank you for the suggestion and will report here the results. Al.

Hello again, Rol*s; Results for the Power-Braking test:
Cold idle, no smells, equal slight pressure at each pipe.
Warm idle, same as cold but slightly more pressure on each.
Reverse gear, brake on, including the emergency brake, accelerate to about 2,200 RPM's. Got more pressure from exhaust pipes, equal pressures, warmer by now, but still can easily plug each pressure force with palm of my hand. Could hear no miss nor smell anything strong or foul, no smoke, only condensation. All normal. Will try tomorrow to disconnect two small hoses to EGR, plug them and drive it around. Might be a vacuum leak at the diaphragm. Will report again then. Thank you. Al.

Hello Rol*s. Yes, have compared the pressures on both cars. Same on each; the reason is, that there is a short piece of pipe joining the dual exhaust pipes right after the catalytic converters and before the mufflers that when I hold my hand on the tailpipe, the pressure flows out the other. Tried something else: Bypassed the EGR valve (thinking a leaking diaphragm would be causing the only vacuum leak I found with a smoke test) by disconnecting and plugging two vacuum hoses. No difference, except a check engine light code I will resolve tomorrow. Most car owners, even mechanics that have driven this Town Car, would not notice or be concerned about this slight missing, but I am planning a very long trip and want it to be as efficient as it can be. Something else I found by comparing driving both Town Car's is a slight delay when moderate acceleration, say from 35 to 50 MPH. Not the transmission slipping, just a slight lag, no bogging down gasping for air, just a half second to react. Oh, the Vacuum reads 14 PSI at idle, should it not be 15 # or so? Will compare those tomorrow also. Rol*s, you have very logical and sound recommendations. Thank you for your help with this. Will want to help with a donation if you have a way to do so. Al.

All true, Rol*s; Have done all of what you have mentioned. New proper Motorcraft plugs well gaped and anti seize compound, dielectric grease, swapped plugs after cleaning contacts and testing, all very good ignition now. The check engine light is on because my disconnection of EGR to test vacuum there, where it reads low but using the vacuum line to the A/C vents controls, the vacuum is 20 1/2 #, very steady and one pound higher than the 1999 TC at same vacuum port location. Not all vacuum sources are full engine vacuum I discovered. The throttle body, I have taken apart and cleaned it thoroughly, as well as the EGR valve and all other related components, decarbonized engine valves top and bottom (Seafoam through PCV valve and in engine oil). Checked catalytic converters as your article indicated and all clear, even hitting them to listen for loose innards. All good. New fuel filter and air filter, good PCV valve, cleaned the MAF and no vacuum leaks except for a small amount of smoke from EGR diaphragm when pressurized with smoke. So there is nothing left to consider except maybe one of the new Khumho tires may be out of round, as the mechanic where the wheel alignment and full suspension was checked said. Brakes are not dragging either.
Therefore, Rol*s, I am satisfied and confident that this 2003 TC. Is ready for the long road trip planned. I am giving up on this very small perceived miss and hesitation on moderate acceleration. It is a fifteen year old car with 109,000 miles after all.
Again, I am very grateful for your help and sound advise and great recommendations. Thank you.
I will also be trying to help others here with some of my thoughts to try to solve their problems with my replies on this fine site you are a big part of Al.

Glad we are both happy, Rol*s, I will be thanking you again in my mind, when driving long and far on that superslab that we all pay taxes for. Good luck to you and stay well and so helpful towards others. Al.