I found that the HMM-Y mic output was understandably lower in level than the stock electret based mic. In order to accommodate this I performed surgery on my FT-100D, changing R1334 (SMT resistor) from 10k Ohms to 1k, thereby increasing the mic pre-amp gain by about 14 dB.
(I don't recommend anyone doing this unless they have an SMT soldering station, good SMT experience and very steady hands. You can easily ruin the PC board).
I also experimented with "full range" and "DX" elements in the Heil mic, the 100D processor, the MIC DSP settings and the carrier offset controls. Using a monitor receiver with the 100D running into a dummy load I converged on the best setup.
For rag chewing the "full range" element gets great audio reports. I brought the TX carrier injection up to +200 Hz which removed some excess bass response. No MIC DSP is used (i.e. setting = OFF). Processor is off. MIC GAIN setting is around 50 to 60 with the Heil mic and the FT-100D preamp 14 dB mod. Before increasing the preamp gain I had to speak loudly with the MIC GAIN set to 100; no good!
So my search for decent 100D transmit audio is finally over, after several years of fussing around with the stock mic, trying a couple modifications on the stock mic and trying every imaginable combination of settings on the rig (and spending about $100 for the Heil mic and AD-100 adapter).
For contesting I think the DX element or perhaps just the processor with the "full range" element will work well. The DX element has an annoying quality to it, but I suppose you want that when you're trying to scream through a pileup.