I recently happened to have a large number of popular tweeters on hand, so I decided to take SPL and impedance measurements and post the results for comparison purposes.  With the exception of the Seas T25 Excel and the Morel MDT33 (both of which have oversize flanges), all frequency response measurements were taken at 1 meter, no crossover components in place, with the tweeter flush mounted on a 8.5" X 17" baffle, with the tweeter center 3.5" down from the top and centered horizontally.  The two larger tweeters were flush mounted in a 10" X 18 3/4" baffle, 5.5" down from the top edge, and offset 1/2" horizontally. 

Below are SPL (2.83V) and impedance plots for each of the smaller tweeters, and then for the MDT33 and Seas T25.  Finally, plots of the off-axis response of the OW1 and Scan Speak 9500 illustrate typical differences between a 3/4" and 1" dome.

Hiquophon OW1

Hiquophon OW3

Morel MDT20

North Creek D2506-3

Scan Speak 9500

Seas 27TDFC (H 1189)

Usher T-9950

Vifa DX25TG-05

NOTE:  The next two frequency plots can really only be compared with each other, since they are taken on a larger baffle than the preceding plots.

Morel MDT33

Seas T25 Excel

Seas 27TFFC

OW1 45 degrees off axis

Scan Speak 9500 45 degrees off axis