I don't have the GR so what follows is general rather than specific.
Synchron-ised line products were recorded, originally for the VI player, on VSL's old Silent Stage which was famously dry. The term "Synchron-ised" is literal in two ways: 1) it allows the VI samples to be played using the newer Synchron Player and 2) IRs allow them to be artificially placed onto VSL's newer Synchron Stage, which really is not dry at all (although Stage B is less reverberant, see Duality). The "MIR unprocessed" preset basically gets rid of 2). So you've basically got the sound of the VI Series product but in the Synchron Player.
Synchron series products are recorded on the Synchron Stage, so have its reverb baked into their sound. You can't really take it out. This means they're not ideal for use with MIR. However, that doesn't mean you can't, as long as you take a few things into account. Firstly, you're not going to get the same results as with a dry source. Essentially you're putting something recorded in one room into another. So just keep that in mind.
There are a couple of practical things you can do: 1) use close mics in the SP. These have less of the reverberant Synchron Stage sound. 2) In MIR, turn off all the Dry Signal Handling stuff.
Above all, as I say, bear this in mind and don't expect Synchron series products to work in MIR in the same way as VIPro, Synchron-ised or other dry sources. You can actually get some wonderful and really interesting sounds this way, I do it quite often.
This is more @Dietz's area and he'll be able to explain it in greater detail but I hope this helps a little.
All the best
Nick