TO,
I don't think it is you that needs convincing but your syndicate mates.
The DR 400 has better visibility, goes faster for the same HP and fuel burn, easier access through that sliding canopy, more space, better load capacity, crisper handling etc etc etc.
2 Drawbacks; one it is supported by Robin who can be a bit patchy at times and secondly you can not stick an 'N' on the side and get an FAA IR to fly IFR in Europe.
I used to have a DR400 and never had anyone who had flown any of the usual light GA stuff that was not more impressed with it than their AA5s, 172, cherokees etc
So maybe let them have a go in one and let that do the hard sell for you
FD