G'day Mate,
Strange how you mention CV boots, every subie that I have a chance to look inside the engine bay has got torn boots, interesting... Maybe a design weakness?
Having worked with CV Joints and Driveshafts for a good number of years, I can say that thermoplastic boots should perform better than rubber boots in the tear resistant field... But unfortunately I'm not that familiar with Subie driveshafts to to know any options available - Always supplied by the competition...
Not knowing which Forester is yours, I would be really surprised if there isn't a newer boot that fits, so keep looking.
Cheers,
pezimm