I have all the pro instruments so RB4 was a giant step backwards.
I still fire up RB2 for some songs that didn't make it into RB3 and I can say overall RB2 is the better game compared to RB4!!
I have hundreds of custom songs and dlc on my PS3 and use a Samsung EVO SSD. It is slow to load the first time but when Fix it but then its much quicker to load a song than RB4 on the PS4, where loading screens are all over the place.