I'm doubtful that they'll ever increase DM service.
They bought about 20 DM engines. I don't know how many are left in working condition. Because of the restriction requiring engines on each end, you need 8 engines to cover the 4 rush hour trips they currently have.
The mechanical failures of the DM's have been well-documented. LIRR has gotten a pass on what I consider to be the bigger case of short-sightedness. The DM's are needed on only the longest LIRR trips. A pair of DM engines can only cover one rush hour roundtrip per day!
Consider the 5:44 from Port Jeff to Penn. It's scheduled for a 7:20 arrival. You can't then get it from Penn to any of the other diesel terminals in order to squeeze a second rush hour DM run out of it. So by 7:20 AM you have 2 engines (and the coaches between them) wasted for the rest of the rush hour. When an MU set arrives in Penn at 7:20, the LIRR can race it out to Freeport, Wantagh, Port Wash or Great Neck for a second rush hour trip.
LIRR might have the equiptment to add another 2 or maybe even 3 DM runs during the rush hour. Given that it hasn't happened already, though, I wouldn't recommend holding your breath. If you're looking for the day when you see more trains from Port Jeff to Penn than you do from Port Jeff to HPA during the AM rush -- well, t'aint gonna happen.