I have VL6 Light installed on three computers. Two of them also have VL6 Standard installed and and all three have a seldom-used XP installation. I love Light and Standard. Both versions of VL run well on all computers.
I can't say I've noticed faster booting with Light compared with Standard. Nor does the OS itself seem to run faster. None of these computers are real antiques, the slowest being a 1.3 GHz Celeron. The 2 GHz Mobile Celeron with VL6 Light is probably the slowest in overall performance, having just 512 megs of RAM, a slow hard drive (4200 rpm?), and onboard video. However, that computer performs well enough that I'm not even thinking of getting a newer laptop as long as it keeps working. Although the 1.3 GHz Celeron desktop is the slowest processor, the computer has two 7200 rpm hard drives, a separate video card with 128 megs, and 1 gig of RAM. These give it a performance boost over the laptop.
I don't think you'd gain much by replacing Standard with Light. You can always "lighten up" Standard by removing programs, but this shouldn't do much for performance of those programs aren't running, just give you more disk space.
IceWM is less demanding on the system and leaves more memory free for running things; IceWM is the default in Light. I think you could install IceWM in Standard and use that instead of XFce. Standard does include LXDE, which should be somewhat lighter than XFce.
My advice would be to stick with Standard if you're happy with it. I don't think Light would make much noticeable difference.