Switching to a 27T is not going to help much. There are 13x30 and 12x30 cassettes available from Harris Cyclery and the 30T will make a noticeable difference. Your rear derailleur should handle it. These aren't cheap but less expensive than a drivetrain replacement.

An alternative is to fit an inexpensive 9-speed era MTB rear derailleur and that opens up the world of XXx32, XXx34 and even larger big cogs.