As a Hornet (St T) owner and having seen a couple of GT21s in my time I can assure you that they are completely different hulls.
The Hornet bottom is highly warped and tapers off to about 18 degrees . The GT 21 is more of a constant deadrise bottom by comparison.
In 'designer terms' I think the Hornet is more of an art piece than the GT21 . For their length they both carry a similar amount of freeboard but the Hornet has a beautifully flared bow and not a straight line on the boat anywhere .
Bear in mind that the GT21 is one of my all time favs when it comes to the classics.
Now the unpleasant truth ... just my opinion by the way .
I can't recall how many performance deep v's I've driven since the later 60's but the Hornet hull is the worst of all of them. It does not perform like the other 22-24 degree hulls that I've run . If it had another 5 degrees of deadrise it would probably be a different story but as it is I miss going out on a fun wave jumping trip with mine cos you only need to come down a little bit off centre and KABANG !
It is dry however , and with its shallower v is no doubt more efficient hp for hp than a GT21 would be , but I'd not think twice about taking the GT21 out in rougher weather than I currently do with the St T .
Either way , both hulls/boats are pure poetry for the eyes .
Just because something's old doesn't mean you throw it away !