The Shimano Ultegra "GS" (long arm-road) rear derailleur has huge chain wrap and cog size capacity that is much greater than stated on the Specifications.
I've combined a 50, 39 & 26 Crankset with the 10 speed Tiagra 12-30 cassette. This provides the range of a Compact with a 12-26 cassette using just the large and middle chainring. The 39t middle chainring is good to 25 mph at a 100 rpm cadence. The 26t small chainring and 30t rear cog combination can crawl up a 20% slope at 4 mph with a 60 rpm cadence.
The chain-wrap limit of the Tiagra/105/Ultegra GS, according to Shimano, is 37. I'm operating safely, with every gear combination, using a chain-wrap value of 42. It is also possible to exceed the 27t Cog size capacity and use a 32t.
The practical chain-wrap capacity, with a correct length chain, of the road "GS" RD allows the big-big combination of 50t front Chainring & 30t rear cog with a little safety margin and also allows the small-small combination of 26t & 12t without any slack in the chain.
I also used a Sram 11-32 cassette with a compact last year and had no issues with a 32 size cog. This may not work on all bikes, YMMV.