Here's a pertinent excerpt from the tip hardness effects resource page:
There are many factors related to tip hardness that could influence squirt, including: tip density/weight, tip efficiency, contact time, and effective endmass. “Return of the squirt robot” (BD, August, 2008) documents an experiment related to the effects of tip hardness on squirt. A softer tip did seem to create slightly more squirt, but the experiment was not very well controlled (see the article for more info). In general, if the contact time is longer (as is the case with a softer tip), the effective endmass and resulting squirt should be larger (see the rubber-super-ball-tip report as an example). Another set of more careful experiments documented in the Cue and Tip Testing for Cue Ball Deflection (Squirt) video and “Cue Tip Squirt Testing” (BD, June, 2014) seem to imply that tip type, hardness, and height have very little effect on shaft squirt. Among the wide range of tips tested in the video, the harder tips did result in slightly more squirt. This makes sense because the harder tips are denser and heavier, creating more “endmass.” The shorter contact time seems to be less of a factor than the added weight.
For those interested, here's video with the careful testing for tip hardness squirt differences:
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