The experimental setup used by spadout. Image from .
The 9 different rubbers showed a 6.5° range – which seems to suggest that there is not much difference between them (the results were later scaled to give more pronounced differences in the sticky rankings). Although in the realms of gnarly slab climbing a little extra friction might go a long way. However, rubber friction is not straightforward and there is more to rock boots than just the sticky factor.
The coefficeint of friction is not affected by normal force for most materials, but for rubber it can vary. Image from .
Perhaps more important, climbing requires two seemingly contradictory properties; high stiffness for edging and soft sticky rubber for smearing on smooth slabs. The test may be indicative of smearing performance – but is that the most important aspect?
 M. Lindner, Experimental and analytical investigation of rubber friction, University of Hannover, Pdf here: http://fluid.ippt.gov.pl/ictam04/text/sessions/docs/SM2/12086/SM2_12086.pdf
 An interesting discussion of climbing friction: http://www.theshortspan.com/features/friction.htm