Does the VT-d feature on the i7 have measurable VM gains over the i5 on 2013 RMBP?
So there are a bunch of comments about the merit of the question which it seems you e taken the time to look at and assess. I'll jump right in to starting the discussion on upgrading. 

Firstly, to be clear you're looking at a new early 2015 13" MBP (I assume you're not interested in a 15", which can support quad core processors and add a whole new layer to the discussion). 

Apple's tech specs can be hard to track down, but it looks to me like you got the same processors I did. The i7 supports a turbo up to 3.1 GHz and 4 MB of cache, with the i5s boasting 2.1 and 2.7 GHz processors and 3 MB of cache. You're right in that the i7 supports VT-d, but you'll need some fairly heavy duty software to take advantage of it. The answer about VT-d is that it really depends what sort of virtualization software you're using, my guess being that unless you want to use expensive commercial software, Oracle's VirtualBox will be the first to support that (though I have no reason to expect this, or know when). 

This brings us to the next level: is the i7 worth it over the i5 (will there be notable performance gains)? For you (and others with this question), probably. The increased cache, better cache coherency, and outright speed increase will be what you notice. Being that all the processors for the 13" are dual core, I was unable to find evidence that there is any significant difference between hyper threading moving up and down Intel's CPU line (unlikely, they all use the same basic core).