Sorry for the bad news...it's unfortunate that you have to join our club, but I know you will get plenty of advice and support from the folks on this site.
First of all, with your Gleason, PSA score and prostate size you probably have most all treatment options open to you. So, don't panic or jump at the first possibility. HIFU is certainly an option but, as you probably know, it is not FDA approved in the USA. And, despite the hype from the HIFU docs and sites, my understanding when I researched this as a possibility for my treatment was that there were indeed side effects...particularly in the ED area. I guess my point here is not to dump on HIFU, but to make sure you keep your eyes open and recognize that ALL treatment have some side effects. You need to research the options and make an informed decision.
Now that you have the diagnosis, you should set off to learn as much about the disease and options as possible. Walsh's book is a good start and a good primer. My advice is that you also make sure to get some other opinions/perspective re treatment. Your guy is HIFU-man...great, now you have that perspective. You should consult with a prostate oncologist (who has no dog in the hunt re bias for treatment), an experienced surgeon and an experienced radiation oncologist. Yes, it's literally a PIA to go thru that but this is a HUGE decision and you want complete info before making it.
And, experience counts...big time. If your provider has not done 250+ procedures do not choose them to do one on you. Let them learn on someone else.
Finally, re your pathology, you have to do what you feel on this one. Perhaps one or more of the other docs with whom you consult will help you re verifying the expertise of the lab you used. There are some well known experts, e.g. Epstein at Hopkins and Bostwick in Richmond that you could use.
Hope this helps a bit. I sure many others will weigh in. And, if brachytherapy gets on your short list, there are a few of us here who have some experience with that...you can read my journey by clicking the link in my signature.