There is a chart that I've seen circulating here on BF which is useful for giving a visual of overlapping gear ratios.

The poster of the chart is barrettscv, and he's posted before about using the SRAM Wifli cassette with a Shimano drivetrain. If the cage on your rear derailleur is long enough, you could go that route. I have an Ultegra 6700 gs (medium cage) RD on my bike with an 11-28 cassette in the rear and a 34/50 compact in front. IIRC, I could go the Wifli route if I chose to try that. However I am OK with 11-28 at the moment.