I found a PTFE insulated single conductor wire that has a much higher ampacity then standard THHN building wire. In trying to minimize the space taken up in the conduit I wish to use this smaller gage wire that has a higher ampacity. It is silver plated, stranded copper and meets MIL-W-16878/4 and passes UL VW-1 vertical flame test. Example: 10 AWG is .138" diameter 75A 600VAC vs. 10AWG .161" diameter 30A 600VAC.

Good question, and not sure NEC 310-16 covers that(wire ampacity).
Possibly ask your "Authority Having Jurisdiction".
You would likey need to show the code so he/she would be comfortable with #10 for a 75 amp, or a 50 amp range.
Couple of guys here are very good with Code, and probably know off the top of their head. Good Luck.

Don't forget 240.4(D).

I must ask, why do you feel the need to "minimize the space taken up in the conduit"??
How many wires are you planning on in a single conduit?

I will say, a higher ampacity conductor will help with derating issues.

240.4(D) Small Conductors Unless specifically permitted in 240.4(E) or 240.4(G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

Total wire count is not known yet but I plan to use 6x6" PVC Wireway from Carlon suspended from a ceiling to provide power to rectifiers, pumps, heaters and hot plates situated along (above) a 40' foot long plating line. I'll have at least 10 rectifiers, 24 pumps and 20 heaters (plus a lot of peripheral 110VAC outlets). Some of the rectifiers and heaters are 220VAC. Everything is single phase. The Wireway will enable future expansion and changes without pulling cable and provides a convenient means of attaching drop cords to the plating tanks via cable grips. The wireway doubles as a junction box wherein the connections to the drops are made to the wires from the CB Box. The trough is more expensive as is the higher ampacity wire but I will not need to install additional conduit if I can utilize this smaller wire. (not to mention its -67 to 392 degree F temperature range and its chemical resistant PTFE insulation). Due to the length of run and the number of wires in the conduit (Wireway) I planned on using 10 AWG wire (75A) for a peak resistive load of only 40A. Incidentally, Wireway is a 3 sided trough with removable lid (or cover).

The answer to this is simple, no.

PTFE insulation is not listed in the National Electric Code as allowed for use in building wiring.

Refer to the UL Listing for the wire proposed to learn what it can be used for.