Bicycle Mechanics - SS or GS with Compact crank?

My new DA 7800 RD SS just arrived. It appears I misread whatever it was I read when deciding between the SS and GS since the spec sheet that came w/ the RD says the GS should be used with a Compact crank. At least that's my interpretation. :) So my questions are ...

Is the GS the correct RD to use with a 50/34 Compact?

What happens if the SS is used with a Compact?

Thanks much ...

Well, yes, technically, a mid- or long-cage RD should be used with a compact (not necessarily limited to compact, but any front combination with that large of a difference from largest to smallest chainrings). The reason is because the RD needs to be able to absorb the slack in the small-small combination and also allow the big-big combination to run without being too tight. Worst case depends a little bit on chain length. If you have a short-ish chain and you shift into big-big, you could potentially destroy your RD. If your chain is long-ish and you shift into small-small, you might have some chain slack which could cause shifting problems.

Fortunately, though, Shimano has a lot of buffer in their specs, and many people (myself included) find that running a short-cage RD with a properly-sized chain on a compact (50/34) works just fine, even in the "bad" gear combinations (big-big and small-small). It's just that the margin of error is reduced, and you may have trouble if you switch cassettes or whatever.

I'd say: if you have the time to wait and the place you bought it from has a good return/exchange policy, go for the GS. But if you feel you're "stuck" with the SS, go ahead and use it. Just be careful sizing your chain, stay out of big-big, and you'll be fine.

What cassette are you using?

Your crankset uses up 50-34 = 16 teeth of chain wrap up.
Short cage DA rear derailleur is rated for 29 teeth of wrap up.
Long cage DA rear derailleur is rated for 37 teeth of wrap up.

So if you are using a 12/25 cassette, you're OK. 12/27 you can probably still stretch it since you probably won't use the little/little anyway.

I use the SS with my compact and run an 11/23, 12/23, or 12/27 all on the same chain. The trick is, as stated above, to size the chain properly. Also, I do not cross chain too much when using the 12/27 to avoid putting too much tension on the RD. I avoid going larger than the 50/23, although it will technically stretch out all the way to the 27.

Thanks for all the help! I don't need it right away and Excel Sports Boulder has an excellent return policy. I'm going to play it safe and get the GS.