So, can I not use the 'curvy route' option in basecamp unless I upgrade to a 590?
In BaseCamp, running on your computer, it will plot you a route that it perceives to be 'curvey'. How it does it might depend on some of your other settings or it may override them, I have no idea. Play around with it. You can't break it, it's free and they can't touch you for it.
Send the route to your GPS device, which will run it quite happily.
Whether you are happy with the route it offers up, is another matter entirely. If you are unhappy, plot your own or amend it to suit yourself. If you are happy, go for it.