On the whole I think this is a better basis for scoring than a fixed-points system. A number of other championships run this system. The whole essence of speed events is that you are competing mostly against the clock. It's the best way of measuring every individuals performance irrespective of who and how many competitors turn up in each class. Of course you could still get classes where there are 'soft' records, but the L7C champs has become so competitive in the last 2 years that the leading cars in every class are damned well driven.
But, I do think there is a problem with wet events. If the guys fighting for the championship tend to discard enough results for the wet ones not to matter, then wet-weather skills get discarded along with those results. OTOH I don't think adding a % to the bogey time for a declared 'wet weather' event works because it's quite possible that *some* people will get a dry run and hence score a huge bonus. FWIW I think it would be better to revert to the 20pts system if the track isn't completely dry for one whole timed run, or something. Needless to say this decision cannot be taken by a competitor for fear of there being something of a conflict of interest... Ideally the CoC should be briefed and charged with making the decision.
You could compromise and score each class on the existing 20pts basis but score the overall championship on a bogey-time basis, but that would be complicated to score and might (though unlikely) end up with the overall champion not being a class winner.
Mike