Peter,
"Real-world" - that's a tricky term. If I test something over my WAN(s) and give you the numbers will it be also applicable for you? :-)
But I see what you mean, I think the closest we can give in this regard is IMIX testing, wth little-to-no accounting of packet loss or latency.
Strictly speaking 1941 should be within the specs you're looking for (in terms of raw crypto performance), however you need to account for control plane usage, amount of tunnels/routing adjacancies/etc.
2900 series in general will provide a bit more CPU power, which should allow you to easily scale up in future.
That being said I think it's fair for me to say - please discuss sizing/scalability with your Cisco SE or Advanced Services team. There is only so much we can account for in a dicussion on forums.
M.