Hi there. Hope you had a good week end.
When you say that, do you mean from rarity point of view or power-performance.
Sort of both in my opinion but it definately didn't have the power of the Hemi. Mopar and Pontiac worlds are a wee bit different.
Given that this engine was introduced post 1972 when most if not all manufacturers were down sizing engine size and horsepower, the SD 455 was a great engine in the time of emissions, decreased power, and performance.
Here's a quote from http://www.musclecarclub.com/musclecars/pontiac-firebird/pontiac-firebird-history-2.shtml
"Comments: 1973 saw some significant new changes to the Firebird, including a new eggcrate grille and a revised front bumper. Engine ratings were down again, with the 350 rated at a mere 150 bhp and the 400 at 250 bhp. But two things would save the Firebird and firmly establish Pontiac as the performance manufacturer during the dark days of the 1970s. The first was the introduction of a new Super Duty 455 V8 (SD-455). "Super Duty" was the name Pontiac used on its high performance (i.e. race ready) engines in the early 1960's, and the Super Duty 455 was a street legal race prepped engine. All SD-455's were hand assembled, and had a reinforced block, special cam shaft, aluminum pistons, oversize valves and header-like exhaust manifolds. Pontiac rated the SD-455 engines at a stout 310 bhp and 390 lb-ft, but experts agreed that it was closer to 370 bhp. This level of engine ratings were unheard of in an era when other manufacturer's engines' output were falling dramatically, and the Firebird became the only true performance car remaining in the market. Unfortunately, Super Duty engines were expensive and therefore rare; only 252 Trans Am's and 43 Formula 455's received the SD-455 engine. The second major coup for the Firebird was the introduction of a new Firebird decal on the Trans Am, which covered almost the entire hood and was quickly nick-named "the screaming chicken," which added to the excitement and popularity of the entire Firebird lineup. Sales of the Firebird and Trans Am were up dramatically, ensuring their survival.
Production: Formula: 10,166 Trans Am: 4,802
Engines: 350 V8 150 bhp. 400 V8 230 bhp @ 4400rpm, 325 lb-ft @ 3200 rpm. 455 V8 250 bhp @ 4000 rpm, 370 lb-ft @ 2800 rpm. 455 (SD) V8 310 bhp @ 4000 rpm, 390 lb-ft @ 3600 rpm.