So at 170 Meg your looking at roughly 3/4 db improvement over LMR with 1/2 inch. I'd gather some prices if you have not already for 1/2 inch, gather the PIM specs for each, and tell the decision making body that 3 db is half power. It might not seem like much but put it in that perspective.
Another approach may be needed since those that have technical knowledge (or at least know to ask those that do) agree that LMR-400 isn't the way to go, but the bean counters understand that it's less expensive so that's important to them. Let them know that using inexpensive (and lossy) coax will require a more powerful (and thereby more expensive) repeater to do the same job as the lower powered one using the cheap coax. Also let them know that using LMR-400 (with the associated distortion) would require separate transmit and receive antenna systems so to use it you'll require double the cost for the antenna, coax, and installation (perhaps even stronger towers to hold the additional antennas, and perhaps even taller towers to allow sufficient vertical separation for the transmit and receive antennas).
Technical talk to bean counters will generally not give them the necessary information to make a correct decision. Talk to them in dollar terms, something that they can understand. e.g. It'll be $5,000 to do the job right using the correct tools and parts. If we go cheap here, well save $200, but it'll cost us an extra $1700 to correct for the issues that lower cost part will generate. New total would be $6,500.
