I have several of the Buzzes! I take about a third off about now then cut down to healthy new growth in Spring, leaving about 3 to 6" on the stems. They will get bushier every year.
I have had them in large pots for 3 years now, and in the garden for just a year and I do find that they grow much more vigorously in the garden. Also my garden planted Buzzes produced many more flowers, and for a longer period than the potted plants. They were one of the very last plants in flower this October.
The other thing worth noting is that they easily form strong new plants from cuttings. I just nip off a fresh new silvery bud when they are a couple of inches long, and stick it in all purpose compost and they grow away quickly.
Interestingly I have never had any success with the ivory buzz, but the magentas and blues are strong plants for me.