I like the look and feel of the Magento site more than the Volusion site as well, although I'm not feeling the white space at the top. I get why you want to use white space, but this serves no real purpose. You could move stuff up more against the top of the page.
I don't like the functionality of the Magento site as much, though. The big problem is it took me three clicks to see any product. At least with the Volusion site you had the drop down menu to get to products right away (e.g. Dirt Bike parts). That's too much. I get why you want subcategories, but show featured sales and products as well.
The reason Fireproof thinks the new site is more sluggish is because...well, it is more sluggish. You've got a serious server response time issue, although if this is a temporary or home host and not the final host this may well resolve itself. It won't resolve all the other issues, though.
https://developers.google.com/speed/....nextmp.net%2F
https://developers.google.com/speed/...ssgiant.com%2F
On the first one, I saw a response time of 3.8 seconds, which is far too long (this will vary, but you generally don't want it more than about 200 ms if you can get it that way).
Another thing you'll need to seriously consider is this, and I guarantee you there's only one other person on this forum who would have thought of it but it's incredibly important.
Dirt Bike Parts
Dirt Bike Parts - Dirt Bike - Department
Those aren't the same URL, although they're for pretty much the same thing. You
need to find a way to 301 redirect the old URLs to the new ones if you're getting any search engine traffic at all (and I suspect you are), or you're going to take a serious short-term hit and one you may never recover from in the eyes of Google, Bing et. al. Ideally, you won't change the URL structure at all if you don't absolutely have to. If you must, then make sure the 301 redirect is in place...you'll suffer a hit either way, but with a 301 the hit will be much shorter in terms of effect and you generally will recover from it. The extremely oversimplified reason is that older pages have been around longer, you may have generated external links from other sites that are of value, and you don't want to lose that credibility. Another extremely oversimplified reason (and probably the more important one) is that if those pages have external links from other sites, you don't want to direct them The Island of 404 Misfit Toys if you don't have to...give them some content.
So yeah, you're going to want to rethink this quite a bit. The good thing is that you haven't launched it yet.