Let me give you a rundown of what Sai had:

Sai first and foremost was a Medical Ninjutsu specialist that mastered the Jugo Clan Kekkei Genkai and even became a Sage without the training of the snakes or toads. He personally had Water and Lightning Release and used his Kekkei Genkai to unlock Storm Release. The mother fucker literally fired Lasers. After that, he attained the Earth Grudge Fear Hiden, and let's just skip some plot and storytime to say he stole the hearts of a Hyuuga, a Hon, a Senju and an Uchiha as well as several Uchiha eyes. He stole the Nibi, and locked it away (its location is still unknown) then the one, three, six, seven and nine tails by sealing one onto his body and four into masks that housed the stolen hearts. Essentially, at the end, Sai controlled five bijuu on his own, had a weaker version of every Kekkei Genkai that he used simultaneously, was a Sage that constantly absorbed nature chakra and could use every element, plus Storm and Wood Release and on top of [b]ALL[/b] of that still could manipulate his own body via making it into threads and using the Jugo Clan KG. Sai could not unlock the Mangekyou or Rinnegan because he focused more on his Sage Style and because his body was too 'mangled' via Earth Grudge Fear and his KG to actually develop it.

Aside from all of that, Sai was probably on the same level as Madara or Hashirama. The differences being that Sai attained a lot of his power through stealing it or recreating it. Sai had a huge amount of abilities and a huge power level, but if you took away his bijuu and his hearts (as they did in the final battle with him) he truly was more of a jack of all trades that had a sage style and was an expert medical nin. That sounds great, but against a shitload of incredibly powerful shinobi, even someone like Sai was no real match. The fact Amegakure was destroyed parallels canon. The fight between Madara and Hashirama altered entire landscapes, and the fight between Sai and the Resistance destroyed an entire village. I would not say Sai was a lot like Nagato (Pain), but more like Kabuto or Orochimaru.