The rigidity factor is real.
PE boats flex on waves, and even on your shifting weight.
It's really noticeable on my kevlar kestrel how the entire boat moves when I shift my weight.
Sitting still in the narrow hull with thigh straps, can balance the boat by pulling with your knees.
The nice thing about the 39-lb boat is handling it - the weight is always surprising when you lift the boat.
You also notice the weight difference in acceleration, the boat comes on speed right now.
As far as the speed difference compared to my T160 - I can't quite keep up with my buddy's Mirage Revo 16 in the T160, and he can't quite keep up with me in the Kestrel. Unless you're comparing directly like this, can't notice the speed difference except for the acceleration thing.
The weight difference between the two is exactly double.
The really neat thing about the Kestrel is the glide - even upwind, the boat just keeps going with minimal paddling, and seems windproof. The wind and glide difference, you notice in a big way.
In fact, the Kestrel doesn't behave well with a big charging paddle - it oversteers and tests your balance - it responds much better to gentle strokes with a touring paddle. The Kestrel doesn't have a rudder, and doesn't need one. After I got brave on the balance, learned to steer with lean against the thigh straps.
The hull shapes are a big factor between these two boats. What makes the Tarpon more stable also makes it slower. E.g., you can sit with both legs over one side of the Tarpon, but if you try that in the Kestrel, you're sitting in the water - the boat will squirt out from under you. In general, paddling it is much more like riding a bike - secondary stability really comes on with motion.
Between the two, the Tarpon is serious touring-distance fishing, and while the Kestrel is noticeably more efficient, it's more of a play boat with some light fishing thrown in. I also paid $100 less for the Kestrel, but it was the last demo boat, and priced at half-off the last retail price.
Both boats are rated at 325-lb capacity, but the Kestrel is not there. The position of your ballast in the hull affects the handling and balance of the boat - it's happier with your ballast up front to counter your weight - adding weight to the rear makes it less stable.
You notice the hull difference in how close together are your feet
The boats you might compare directly are Diablo, because they're available in both ABS and PE, about 35% weight difference, though the ABS won't be as rigid as kevlar composite.
CD also offers the SINK Kestrel in both rotomolded and kevlar - otherwise identical hulls, which is a chance to compare the two directly. Interesting, the SINK Kestrel in kevlar composite is 4 lbs heavier than the SOT - I guess the double hull effect of SOT geometry adds stiffness and can save some weight.