Almost every time I've flown out of STT, the queues were long and slow. And there are 2 consecutive lines, one for immigration and one for baggage. Without baggage, you could skip the shorter line

So if you assume the ferry is on time, and you are at the front of the line for USVI immigration at (Charlotte Amalie or Red Hook?), and you catch a cab immediately, and the check-in at STT was done online, and you only need to join the end of the line for immigration -- then you'll have time. But if more than 1 of the above doesn't work, you'll start stressing...