if i can afford the bike, i should be able to afford to replace the drivetrain. i don't live in an area where we get a lot of super heavy muddy races though.
if i lived in portland or seattle, i'd race on a SS for sure.
No. I would use the extra pop for a second identical bike. Unless you are sponsored there is no benefit to DA over Ultegra and if you are running 10 speed I wouldn't go above 105 for most. It is going to get shredded ... save the DA for the road ... just my 2 cents