I recommend that you find out where you are by mocking up a valve, retainer and locks without the spring and measuring the distance from the bottom of the retainer to the seat. You're assuming that the Comp retainer has the same dimensions as the stock retainer and that may not be the case at all.
Even if they are the same and you actually do have 0.030" less installed height, multiply the spring rate times .030 and find the new seat pressure and open pressure. Example, 370 lbs per inch times .03 = 11 lbs. In my opinion, that wouldn't make or break the deal as long as you have no coil bind and the retainer misses the guide and seal at full lift. As far as coil bind, you want a minimum of 0.010" between each of the 5 coils. As far as retainer/seal clearance, you want 0.060"-0.120" at full lift.