This seems reasonable, but it's not quite that straightforward, especially for Frost. You gain buff uptime, but it's not a direct 40/25 times as much; sometimes you get a proc when you already have the buff. And RE has other benefits. Not only does it give you slightly more runes, but it can give you better runes. So you're probably right, but I'd like to be sure.
While Rune Regen talents can certainly be said to be a choice at Level 90, the Tier 13 four set bonus makes for a clear winner at the time of this writing. Runic Corruption yield the highest uptime (due to increased proc chance) for the four set, and should thus be defaulted to by pretty much all DPS DK specs. Blood Tap has its value further diminished by not interacting with the four set in any way, as well as our toolkit at 85 lacking Soul Reaper (the uptime of which is where Blood Tap truly shines).
I wanted to put your theory to the test by running some sims to make some pretty graphs. Unfortunately the t13 4 piece set bonus seems to be hard coded in simulationcraft. If you're frost and have RC, the sim knows that's impossible (since the talent wasn't available before 5.0), and while you do get the correct rune regeneration, you won't get any set bonus uptime. It was quick and easy to verify in game at a training dummy that this is a bug in the sim, the set bonus will definitely proc as frost spec with the runic corruption talent.
However, before I request a new build with this fixed, I think further dummy tests are in order. The original intent of the set bonus was for unholy to get a 40% proc chance, and frost to get a 25% proc chance, to compensate for the different runic power costs of death coil and frost strike. So does the set bonus work as written, with a proc chance that varies based on your selected rune regen talent? Or does it work as intended, with a proc chance that varies based on your spec?