The geese had different motors, too, so I'd think the sound would certainly be close enough to count. Let's be practical--lacking a recording of a Mack engine, any small gas engine will have to do in order to be reasonably prototypical. It's certainly going to be much closer than an Alco or EMD prime mover.
I've got Soundtraxx Tsunami decoders in both of my Geese. I'm using the 1-amp version, and that's proven perfectly adequate. Note that you can find the older 1-amp Tsunami decoders pretty cheap now that the Tsunami2 decoders are out on the market. The differences between the older and newer generation boards aren't really noticeable to the operation of the Geese or other critters, so if you can find one for cheap(ish), you're not losing anything. I've seen them going for as low as $50 from dealers looking to clear old stock.