Mike is correct in that I'd feel a lot better about doing an actual deflection test because of the composite layup in order to do any mathematical predictive stuff with these. From an engineering/materials testing standpoint, the equations work very well to predict deflection values for different geometries (i.e. sizes and shapes) of materials that are basically isotropic (carbon, fiberglass, metals, even wood if you understand or accept that the standard deviation on wood will be much higher than on composites or metals.)
That said, Sky Shark tubes are composite tubes, made up of both carbon and a wrap of fiberglass. Or even pultruded carbon wrapped with a woven carbon fabric for increased hoop strength in lieu of fiberglass (which appears to be the case for the P2X and P3X tubes.) Anyways, because you have differing amounts of each part of the composite layup between tube numbers, it makes predicting the deflection of a "new" composite tube very difficult, and empirical bench test data is best. Actual data is always best anyway, but I've found that the predictive data for known common materials (carbon and fiberglass) is very good and almost dead on, so I feel comfortable using it to "expand" the chart where certain sizes were omitted or were not tested originally (the 0.2810" OD pultruded carbon tube is a good example.)
HOWEVER - if the question is simply "which one is stiffer" in the 8P vs P450 in a simple, relative sense - I've asked John T. about these and the 8P is stiffer. A set of deflection test data would nail down the how much
stiffer and scale factors.
It is interesting to note that there is some debate on the accuracy of the P400 "standard deflection" that was in the original data set on the original chart (before I got my hands on it LOL) I've read (elsewhere, can't find it at the moment) that the P400's tested and measured at 0.302" deflection were of a lighter modulus of carbon.
Anyways - when I was measuring a couple of tubes that I wanted to know more about that weren't on the chart, I set up a little deflection bench test for those and while I was at it I tested a handful of the P400's that I had on hand. They were very consistent (as expected) and their average deflection in "standard" values was 0.227" with a range of 0.224 - 0.230. These
measured values are more consistent with the reality that P400's are in fact stiffer than P300's (flex a couple and you'll see that, relatively speaking, P400s are stiffer.)
I'll pick up a couple of P450's here in a couple of weeks and test 'em out and see where they land; I *think* they'll be a little stiffer than P400's, but the 8P will still be the "Big Kahuna" for the Sky Shark straight tubes.
For what it's worth....