Hi California,
I'll take a shot at this question. Keep in mind the '83 GS was a HUGE rage because it was designed for fast revving, race type power. It was also considered one of the best handling!! This was right before the advent a 'Interceptors', 'Ninjas' & 'Hurricanes'. The dyno doesn't always show an accurate picture for 'where' the power is hooking up, but gives good indication of how much, when it peaks, & when it fades. The Dyno on the 450 I just looked at showed peak HP of 35.4 @ 7500rpm.
The Magna was different than the Sabre (rode a Sabre & almost bought one ...fast, but not a great handler). The Magna cruiser was WAY different in the engine compartment, BUT, that 300 extra CC's shows it's low end grunt. The Dyno said 54.8hp @ 5000rpm. The Sabre made 65hp.
In a shoot out, I'd take the Suzuki any day! In a long ride, give me the Magna! A lot has to do with the head, porting, cam, valves, & how freely the bike can rev (like inline vs v-twin).
Interesting question ...good to remember the 80s motorcycles when things were changing fast. Do you remember the 650 TURBO?? The Yamaha Seca 750 Turbo scared the poo out of me (I was 17!).
Hope this helped (and I read numbers right).