I just tested the new version of ANTLR that uses ST v4 not v3. In terms of code generation, it's just about twice as fast given plenty of memory (750M). To process Jim Idle's 15296 line TSQL grammar, it takes 1760ms instead of 3624ms, though that doesn't alter the overall wall clock performance much. It still takes about 12 seconds to process the grammar. It generates a whopping 168,677 lines of Java code (not including the lexer). That gives us about 95,830 generated lines of code per second versus 46,540 lines per second previously.
Given more restrictive memory, 175M, it's a completely different story. The wall clock for ANTLR with ST v4 on TSQL is only about 15.5s vs 24s for the version with ST v3. Looking at just the code generation component, ST v4 generates code in 2667ms vs 11394ms. It's about 4.2x faster with restricted memory This can matter if you use ST in a larger application that needs lots of RAM. I took the ram up to 230M before the ST v3 version was able to complete without speed problems from memory pressure.
So, ST v4 seems to be about 2x faster than v3 when there's no memory pressure / GC thrashing. With memory constraints, ST v4 is much faster because it seems to use less memory.