Here are the charts comparing the frequency responses using the 225 as a base and changing out the pads. The first graphs is for the stock pads and the second shows the very large G-cushion pads.
http://www.innerfidelity.com/images/GradoSR225i.pdf
http://www.innerfidelity.com/images/GradoSR225iGPads.pdf
I understand your point about frequency responses, but the variations on the Grado headphones I posted in the graphs are quite small considering the pads are the primary difference. The HD650 and HD800 for example have different driver sizes, resonators, and very different frequency responses...upwards of +/- 15dB. So comparing Sennheiser's HD650 to their flagship HD800 shows more variation then any of the Grado headphones. Grado on the other hand is selling the SR-60 for under $100 and RS-1 for over $600 and all that I feel you gain is the wooden earcups. The other headphones you list
do sound very similar and I have owned all of them. The Grado line varies up to $500 for virtually the same sound signature.
I see no reason to spend more on a Grado, in regards to sound quality, then what the SR-80 has to offer.