You can cut back hard now! You won’t get any blooms, but you’ll get more growth.
When they develop satsuki in Japan, they don’t let them bloom for 10 to 12 years. They use the energy the tree would waste on flowers to build trunk and branches.
You can cut back now an it is true you won't get flowers, but the rest of that story is not completely accurate.
Adair and I agree to disagree on this, but you will get just as much growth if you cut it back after the flowers as you will if you cut it back before it blooms, thus removing all of the flowers. And, if you do remove the flowers in a cut back, it really won't take off that quickly, you will maybe be only a couple of weeks at the most ahead. Once it sets buds and prepares to flower, it is in that mode, and it still takes it a while to start putting out new growth. The plant has the energy to flower
and put out new growth. It is not gonna grow any more or less if you remove the flowers. There is no mechanism inside the plant that says, "hey, I have three times the energy since I don't have all of those flowers so I will now put out three times the foliage, and three times as fast." In fact, I think it makes them a little sad, since they can't show off what they worked the whole previous year for. They grow like weeds anyway, you will be pruning and wiring new growth all summer and get to enjoy some flowers. Let it bloom!
All of this is nothing I have seen on the internet, heard, or read in an outdated book. It is from personal observation from years of growing azaleas.