Graviman-
The results from that 1985 Garrett project were pretty impressive indeed: 2.3 hp/lb wet installed and an overall BSFC of 0.33 lb/hp-hr.
Here's a link to another paper of a trade study Garrett did. On page 7 there is a comparison between notional 1000hp turboshaft and compound engine drivetrains operating at 4K95F conditions. Over a 2.5 hr flight the combined weight of the engine, fuel and fuel tank is 16% lower for the compound engine.
The Garrett project results are even more impressive when you consider that all of the testing was done using circa-1985 commercial mechanical diesel injection equipment. The DDC unit injectors used could produce injection pressures around 1200bar, but they did not provide accurate control of injection timing or duration. As you pointed out, just imagine how much better this compound engine would have performed using a modern 3000bar, high-frequency, direct drive piezoelectric, digitally controlled, common rail injection system.
As for your concern about the durability of such an injection system, they are used widely on modern production automotive diesel engines and provide extremely reliable service for well over 150K miles. 150K miles of highway driving would be roughly equivalent to 3000hrs operation, which would seem to be acceptable for current commercial rotorcraft service.
It would be great to see someone pick up where Garrett left off with this concept.