A 2300 RDER is listed as having a 26T max cog size.
A 27T cog will "probably" work fine, but you "might" be pushing things with a 28???

IF this is an 8 speed, you may find your cassette choices very limited. It's hard to find a larger "big" cog without dropping to an 11T cog for the smallest.
This means you'll probably have a nearly unusable highest gear.
One option would be to install a triple crank/rings such as 28-38-48.
This would likely require a new FDER along with the left shifter.
Your RDER is rated at 35T chain wrap capacity, so assuming you have a 13-26 cassette, you would only be using 33T of that. (34T if you have a 12-26 cassette)

http://techdocs.shimano.com/techdocs/index.jsp