Hi Audiman,
We all have different opinions and tolerance of risk. There are people who seek alternative solutions and there are people like me who just want it cutting out immediately.
When I had skin cancer (non-melanoma) there were alternatives to surgery but the dermatologist said it will almost certainly come back and advised me to have it cut out for a permanent solution.
HIFU seems to be similar. Next time it deteriorates you can't be sure you'll be offered surgery or radiotherapy. It might be incurable.
That it's grown 8mm is a warning sign. The volume of a 24mm diameter sphere is 3 times that of a 16mm sphere. It might not be a sphere though.
It often seems the fittest people have no symptoms and some find themselves going too late and not being offered an operation or radiotherapy.
If I was you I'd make a choice and get it done. For me it was the op as it's over in 2 hours and with luck there is no more treatment. With prostate cancer though you're never free of the thought it might come back especially with the frequent post treatment blood tests.
The above might sound a bit doom ridden but Prostate Cancer operations and radiotherapy have good results.
Good luck,
Peter