In the past 1000 miles all over SoCal, I've found that generally I don't need the 11-cog....In fact I use it exclusively when I'm going downhill. However, when I go uphill I find I'm pushing myself. As I'm trying to be more consistent on the cadence, I find that 25 simply isn't enough for me when I'm tackling hills.
I currenyl have this bike with a 11/25 cassetee running on an ultegra derailleur.
Looking around, it seems that 28-cog should be a safe move in that it will work out of the box. 12/30 is possible, but it depends on the bike (hence why I put mine in the link). Is there anything larger than that that would be reccomended or am I looking at more than simply replacing the cassette for ~100 bucks?
I looked at BikeCalc.com - Cadence at all Speeds for any Gear and Wheel and it seems that there is about 5-rpm difference between -28 and -30 cog sizes for the speeds I'd be typically going up hill (5-10mph).