That's not true actually. Waqar at his peak was as fast if not faster than Shoaib, and many of the players who played with both Waqar and Shoaib attest to this fact. He was express pace (90+ average) but after his injury he struggled to even break the 90s barrier. Anyone who saw Waqar bowl afterwards knows he couldn't beat the batsmen out of sheer pace like he used to. And the fact that his injury can account for this can be seen in his dramatic and almost immediate loss of form after his back injury in 94/95, which coincided with the end of his peak.
I see. Well most accounts of Waqar I've read tend to suggest he was no more than "normal" fast bowler speed - certainly not proper lightening fast. However, there were no speedguns in his heyday so we won't ever know absolutely for certain. One thing's for sure - the prodigious late inswing he got will always make him both look and feel faster, if perhaps not neccessarily by all that much.
But every indication is that Steyn seems to be learning how to be a better bowler, as he showed in Australia
I don't doubt that. However, regardless of this, there are limits to what he can achieve. He has no great height and bowls with a low-slung action to minimise what height he does have. Therefore his trajectory is always going to be low, and to attempt to change that would be enormously dangerous, as a wholesale overthrow of a bowling action is. And I don't even think he needs to - the low trajectory has its advantages from a wicket-taking POV.
and his outswinger doesn't seem to be going anywhere. It's rare for any major bowler to lose a natural stock delivery.
I certainly don't see Steyn losing his outswinger in that he suddenly just stops being able to bowl it, but the unfortunate nature of swing bowling is that there are times when, sometimes for not apparent reason, the ball just refuses to swing. It's incredibly annoying (IMO), but there's no way around it.
Steyn generally does about all he can to get the ball to swing - his seam-position is and always has been absolutely beautiful - but nonetheless there will still be times, perhaps not all that often, but times nonetheless, when it doesn't swing for him. Under such circumstances, he's always going to struggle.