UPX doesn't make any program faster. The only point of UPX is saving disk space. However, unless you're talking about small portable removable media, the size of your programs is completely unimportant. You won't save a bit of RAM through this. The decompressor takes some overhead, even if it's just a little. The decompressed executable must be kept in RAM or paged to disk. This is unnecessary with an uncompressed executable because it is already on disk. So where's the significant advantage? I don't see one.

As far as I know, Java bytecode is compressed anyway and even if you saved a few percent of RAM by UPX, the amount of memory used by the Java VM ridicules that effort.