Cassette with compact crank

I ride a compact crank with a 12-25 in the rear.  It works well for anything short of a steep mountain ride and I don't spin out of gears until around 30-32mph.

I believe that SRAM makes an 11-28 cassette but I haven't tried it.

Hardest gear won't change as you will be in 50-11 with either cassette.

The big difference will be in your easiest gear 34-23 vs. 34-28....pretty significant difference as you are gaining a gear 5 gears easier and what you use climbing hills.

Other difference is on flat or rolling terrain you will lose some of your middle gears as spacing is more spread out and skips certain gears. To me this is off set by the positives of gaining easier gears for climbing.

Go for the 11-28, you'll enjoy the extra gears in the hills.

Not sure about the SRAM version, but I have the Ultegra 6700 11-28 and the only 2 gears different to the 11-25 I used to have are the biggest 2.

The 3 biggest gears go from 21, 23, 25 to 21, 24, 28 which for me means it's a no brainer.

I also kept the same chain, and had no problems!

Hope that helps

Nigel

Probably not, unless your chain is badly worn & near replacement time anyway.  But no harm in trying the larger cassette with current chain.  For various biking events I swap between 11-23 & 12-28 depending on terrain, and across 4 road bikes.  Never had a problem.