There - now you have shown that Icom support do know the answer to that question!
Still looking forward to getting my 421, ordered nearly a week ago so should arrivev soon. Spent a hour Sunday knocking up a facia mounting plate for it to go over the existing Simrad (larger) sized hole!
You will of course get a review (good or bad!) including ease of conection to gps (I don't have Navman!)
Bear in mind that the Silva S15 (I don't own one, thank goodness, but I know a man who does) can't be left in Navtex mode overnight, as it's impossible to disable the DSC alerts, so you get woken every time a DSC call is received.
From a design perspective, that lack of attention to detail is a shocker!
I've got a NASA SX 35 and an non-DSC ICOM on my mobo, with the 601 on order to replace the NASA (which seems to be rubbish). I like the auto fog signals and loudhailer on the ICOM, plus the fact that it's a big, solid, well-engineered unit, and I've been very impressed by my smaller ICOM.