More than likely a Kirkham or original style chassis is P-L-E-N-T-Y stiff for us mere mortals. Consider what the originals did, or even CSX3170 and his recent autox successes.
Besides, IMO it would be tough to quantify how much better performing a "600 LB stiffer" frame would react unless you A/B'd them with the same motor, driver, etc. Like I said in another thread...until it's been done, it's all bench racing and fairy tales. Personally, I'd rather have a 2100 LB accurate chassis Cobra than a 2600 LB stiffer one. And as far as where the added weight
comes from, it's more in the glass probably. A glass CSX is in the 2500-2600 LB area, no?
For what it's worth, I remember sticking the tips of my fingers between the door and cowl of my old SPF while driving. Judging by the amount of pinching I felt compared to the same "test" with my Kirkham, I'd say the Kirkham is quite a bit stiffer (much less movement). I know, it's not scientific
That's no dig to SPF. They're fantastic cars.