LIke all NAP heads they sport nice quality razor sharp blades.......really nice blades on the Spitfire.

I only have one deer shot with one and it was a very short track job but yes it was probably the smallest entry wound I have seen on a any deer I've shot with mechanicals which is darn near 30 or so animals.
The retention clips are quite stiff and it takes some force to get em opened which I'm sure along with the rounded blade tips contributes to the delayed opening and incompatibility with lower energy set ups.

I would feel confident using them on game with my 70+ft lbs of KE but for consistent pass throughs I don't think I'd want to be too much below that level.

To tell you the truth, if I were to use them I would possibly remove the retention clips entirely and use a rubber band style blade retention ala Rocket to increase the speed that they open on entrance. I wish Rocket heads had blades comparable to Spitfires for sharpness that's for sure.

If you buy a pack of them make sure you take each head out of the package and check each and every blade for the force it takes to open. If any seem especially sticky make sure you try to tune it so that they all open together and with as little force as possible.