While its not in YC - there are a few SAAS type security startups aimed at SME's - such as https://intruder.io/ who I am a very happy customer of. This to me looks to be high growth.
Personally I don't think any serious pentester will spend considerable time in this model though.
That said, I also think that one of the main challenges for buyers of pentest services these days is to evaluate the quality of a report or of the work done. Thoughts on how to improve that are most welcome.