Larger cassette options on a compact set up?
This ships with a Tiagra SS. 

Derailleurs come in 3 lengths: SS (narrow range) , GS (medium range), SGS (wide range). 

If you want to move past 30t, you need to switch to a SGS derailleur, likely from the Shimano mountain range (e.g. Deore M591) - its 9 speed so the cable pulls are the same between the 9 speed deore and 9 speed Tiagra. This is what I'd likely do if i wanted a bigger cassette.