I did a quick(ish) test of power perception comparisons in prime and the test realm to look at the mana levels as well as the experience it would yield.

For both realms, I did a circle of Shard, not entering the new bridge area, concentrating in each room, and then allowing to drain while on the north wall. Unfortunately, I failed to copy my prime ranks at the start of the power walk, but it was close to where it was once the walk ended and the drain. Here's my data:

Stats: 26 intelligence, 27 wisdom

Prime Results
Starting PP Rank: Roughly 108.55%
Experience at Drain Start: 108.57.31 bewildering
Time Passed: 42 minutes
End Experience: 108.68.68 thoughtful
Total gain: ~11.39% of a rank

Test Results:
Starting PP Rank: 106 29% clear
Experience at Drain Start: 106.33.07 Scrutinizing
Time Passed: 51 minutes
End Experience:106 55.82% thoughtful
Total Gain:~26.82

When I get a chance, I'll try to post an analysis of the different drain rates.