As many of you know I am looking to eventually teach piano (I already teach clarinet with a very high level of success). I'm confident that I could easily teach beginner to intermediate level on the piano as well. However, I'm wondering if it is best to wait until I have my grade 8 in performance before embarking on actively finding piano pupils?

I am actively looking to complete the CertPPTC course and join EPTA as a piano teacher once I have my Grade 8 and spend many hours researching about the piano (repertoire, instruments, pedagogy etc.) Something I can't say about the clarinet.

Piano is much more of a passion than clarinet and I play and explore much more piano repertoire than I do clarinet. However, I am a 'better' clarinettist (diploma level, very, very little repertoire feels too hard for me to have a go) but I really have a desire to teach piano as well! In fact I'm much more interested in piano, just don't play it to the same standard as the clarinet.

Should I just go for it? Use my teaching knowledge gained from teaching clarinet combined with my recently much improved piano technique and start teaching?

See what your teacher thinks.
I'm teaching clarinet some time in advance of having grade 8.
When I was asked to do so I spoke to my teacher who agreed I should do it.
I started teaching piano before I had grade 8 too.
In both instances my teachers agreed that my own technique was secure enough and that I had sufficient understanding.

I would say that if you are as enthusiastic as you sound about the piano you should go for it. Your teaching skills from the clarinet can be transferred and many teachers have no more than grade eight piano-myself included. I think a teacher who is passionate about the subject they are teaching is more than half the battle. I always give prospective pupils/parents a list of my qualifications so that if they are looking for a teacher a higher level of skill than I have they are under no illusions.
Good luck.