Manny never had power at welterweight. His only 2 "knockouts" (and that's in quotes for a reason) at welterweight were against a washed-up Oscar De La Hoya (who retired on his stool) and Cotto at a catchweight below the welterweight limit. Given that Oscar is the only guy who was at the limit and he decided to quit because he didn't have it anymore, in one respect Manny has never actually knocked out a guy at welterweight. Also, he hasn't knocked out anyone in 6 years. Not even a TKO.
At featherweight and super featherweight Manny had great power. At welterweight not so much. And as you said, he's not what he once was. His warrior style - which everyone loves - doesn't typically lend itself to lengthy careers. He's a step slow and the punch output isn't what it once was. I wouldn't call him a pale shadow (Roy Jones Jr. is the definition of that!) but he's simply not elite - at least not with one arm.
Manny still had plenty enough power at Welterweight. Obviously, not nearly so overwhelming as when he was fighting in his more natural lower weight categories, but I had in mind peak Manny of around 2009, 2010 when he destroyed the much bigger Cotto and Margarito.
Sure he fought Cotto at the catchweight of 145 instead of the usual welterweight limit of 147. Big deal. He still beat Cotto up, and Cotto has said that Manny is a much harder puncher than Floyd. “Pacquiao dominated the fight, knocking Cotto down in round three and round four, before the referee stopped the fight at 0:55 of round twelve. “
In 2010, he fought Margarito for the light middleweight title (normally 154 limit), at the catchweight of 150 (still greater than welterweight). He beat Margarito up. Period. He had plenty of power.
“In the penultimate round, Pacquiao implored referee Laurence Cole several times to stop the fight as Margarito had a swollen face and a large cut beneath the right eye, but the referee let the fight continue.Margarito had to be taken directly to the hospital after the fight, where it was discovered his orbital bone had been fractured; he had to undergo surgery. “
So, sure, power-wise, he wasn't as overwhelming at 147 as he was at Flyweight (112) up to Lightweight (135), but he still had enough power to beat much bigger, highly regarded guys up at Welter.
Anyway, it's a shame the fight didn't happen back in 09 or 10. I think Manny would have had an excellent chance to win it back then. He was that good, but he has fallen off way more than Floyd has.