One thing that most racers try to avoid is large percentage jumps between the cogs. You can't have a large range and close cog spacing at the same time, but using a compact will increase the gearing range, so that a slightly closer spaced cassette can be used and produce the same range.

A 53/39 with a 12-27 isn't very racy, except for the mountains. A similar compact crank setup would be a 50/34with an 11-23, not an 11-25. The 50/11 top gear is slightly bigger than the 53/12.

The easiest way to compare the 39/27 ratio to the 34/23 takes some simple math: 27/39 x 34 = 23.5. What that tells you is the 34/23 is not quite as low as the 39/27, but it's close.

There is no free lunch. You pay for the closer spaced compact setup each time you make a shift between the chainrings. After dropping to the little ring, you always have to make one more cog-shift to reach the next ratio in a uniform progression. You'll need to shift 3-4 cogs smaller instead of 2-3 cogs with the 53/39.