On average, will it buy me a significant HD space savings if I'm using high bit rate encoding?

Yes, That's the point of VBR, to save disk space. I don't know what you would consider "significant" though. You didn't say how your current collection was encoded? Give us that information and we can make estimates from that.

Keep this in mind: It's easier to upgrade the disk drives on the player than it is to re-encode your collection. So if your collection is already at a decent bit rate with a good encoder and not too much high-frequency rolloff, then you might not want to bother.