I've used under-tape gel pads before-- Fizik, maybe?-- and was unimpressed because, as I learned the first time, if you don't place them in the high-pressure areas of your grip, you don't get optimal effect, and when I did give a little more thought to placement the second time and put them under the hotspots, you only get the benefit when in those specific grips and the rest of the grip feels differently, or uneven in terms of thickness.

Therefore, I'd suggest going with the tape thickness you prefer uniformly, or if you really want squish, double-wrapped as suggested above. I use LizardSkins DSP 3.2mm on two bikes, and think it's the best feel I've ever had in bar tape in terms of give and texture.