I've read good things about growth rate and current with payaras(vampire fish) and do use a powerhead with my Raph. Never used a powerhead with P's but to put some quick size on my juvie Rhom - I'd be interested in the results.
Funny, my Piraya and Reds generally love current. There are two ideas of current. One being that a good current will maintain healthy fish and the other, excessive current creates monsters.
I've raised two batches of Natts. in excessive current, high temps 82-84 degrees and they were the largest I've seen for the most part. The last batch I grew a Natt. to 11 1/2'' in just over a year. He was in a tank with a total of 2500 gph. This fish was 3'' thick and growing.
With that being said, the set up I posted is simply too much. I tried to provide breaks in the current but there really were none. I took the 2600 used as a powerhead out and put in a 850gph pump which is much better imo.