There are a few options for improving the sound, whilst still retaining the factory speakers. Allthough the stock speakers aren't the best, there are a few of them.The main issue is that they are all running in full pass. This limits both their power handling capabilities, and their overall volume.Even average speakers, like the one's found in the VE, can be tuned and setup to perform quite a bit better than they currently do. In saying that, even a good set of splits running in full range, even if they are amplified, "can" make little to no difference also. Think value for money!
Upgrading the speakers just on their own wouldn't really bring much of an improvement. Aftremarket speakers (depending on what you went for) would most likely be slightly more efficient, but still won't offer much performance for your money unless you accompanied them with an amp, so you wouldn't be getting good value there. A good set of speakers really only begin to perform at the best above a certain fq, and again depending on their sensitivity rating, with a certain amount of power input. But there is no point in driving arond with your fronts at 100hz, and having no bass to back them up. Making a small improvement to all area's, is better than making a large improvement to one area. (unless you have future plans beyond this, and are simply buying one component at a time)
I think its best that you add to your budget, a cheap 4 channel amp. Not really for the point of providing more power to the speakers, but for added tuning capabilties. Running your stock speakers on a HPF, will undoubtebly bring a noticible difference.
Matched with a suitable sub and amp package, there is decent sound to be had. Also keep in mind that the VE is a new vehicle, it is therefore much quieter also. This brings an overall improvement in all sound fequencies, and it also means that at a later date, that a decent set of speakers is just a drop in replacement away, from a complete setup.
OP - There's point in saying (depending on the price) give us your budget, and we can tell if if it's worth while or not. A sub and amp can be added pretty cheap, BUT, using a cheap sub, can sometimes be seen as poor component choice, this relates especially to the size of the vehicle. Sometimes your money can be better spent elsewhere, with more gains for your money spent.
Allthough most outlet's wont tell you this, sometimes it can be better to get the most out of what components your allready have, rather than replacing them with something of better quality, that your unable to make perform at thier best. For e.g, $150 worth of Dynamatt to your front doors, is likely to sound better than $150 speakers in your front doors, as a one off purchase.
Some may dissagree, this is only my opinion. I do not own a store, and i am completely unbiased. Free advice is worth what you pay for it.