The Michelin Country Rocks are going to be slightly slower than a 27" road tyre due to their extra width and lower maximum pressure. They're also heavier, which will cause a speed loss going up hills and lower your average speed.

Another alternative to switching chain rings/cranksets is to change the gear ratio on the cassette/freewheel. A 12T small cog with your 48T chainring will give the same ratio as your current 13T with a 52T chain ring.