Prime for prime? SSM would have gotten the tuneup of his life. Lara's reach and foot speed, timing, accuracy, precision, and counterpunching would have made a mess out of Mosely. Think Vernon Forrest-SSM but 10x worse.
Woulda went similar to the winky Wright fights only worse with the movement. Shane was a small welter let alone jr middle.
Well, yeah, there's really no such thing as "prime vs. prime" for this match-up @ 154lbs. Lara is a 154lber through and through. SSM was beyond half-life by his stint there. Stylistically, you're right, Erislandy is problematic for old chipmunk-cheeks, but if you shrunk him down into a welter or lightweight it would be very interesting to see how that blazing speed and power offset the gap in skill...
With what Lara has shown us in his last two fights, I think it's safe to say that would be one hell of a ****ing war. SSM would probably be the only guy faster of foot or just as fast of foot as him and definitely would have got inside at some point and sropped some massive payload bombs. Without the reach advantage Lara would have had to dig down and dust it up.