I recently purchased a VPN called "ExpressVPN", and if I use this VPN, does it mean that I can get even MORE bandwidth than the internet package I'm signed up for because I'm connecting through high-speed VPN servers
Almost certainly not.
The speed you pay for is the delivery rate to your house from the local ISP network. It's how big a pipe you have into the ISP's fabric.
Then, the ISP network has to deliver your outbound packets and then give you the replies, and vice-versa. Unless it's a very small ISP, they will have multiple connections into the Internet, going to different providers. When you stream video from Netflix, then that data will normally go over one specific pipe, the route that's logically shortest.
That is, say the ISP buys bandwidth from AT&T and Level 3. When you start your communication with Netflix, AT&T might say that they're six hops away, where Level 3 says eight. If the two exit nodes are at the same distance from your origin point, then the traffic should go over the AT&T link, because that's shortest. (there are advanced things the provider can do to change how all this works, but from a 30,000-foot overview, that's how the path is generally selected.)
This means that all the data coming from Netflix is going through many hops from them to you, and the overall rate of data delivery will be constrained by the slowest link in the chain. Typically, internal network fabric for most ISPs is not loaded that heavily, so there are four spots that are the most likely to be the bottleneck:
Why are those last two the usual trouble spots? Beause in case #4, you might not want to pay for enough bandwidth for everything you're doing, or maybe you can't buy enough. The same exact thing applies to case #3, where the ISP may choose not to pay for a fast enough link to AT&T to carry all the Netflix traffic, because those links are their overhead. That's a big expense for them, and many ISPs try to cheap out, not really buying enough transit bandwidth to support all their customers at prime time.
So, sometimes, if you use a VPN, you can get much better performance. Why? Because the ISP sees the link from you to the VPN provider. They can't tell that the packets are then destined for Netflix or wherever, all they see is the encrypted wrapper going to your VPN server. If that connection goes over a different, less-congested Internet link, then you can potentially get much better performance.
In other words, let's use the example above. IF the AT*T link is congested, your Netflix performance will suck. If you luck out, and the VPN provider is fastest over Level 3, then starting a connection with them may move your data to the "good" pipe. Even with the extra overhead of a VPN, if you can jump to a non-congeste transit pipe, that can improve your performance a very great deal.
Netflix is very unpopular with ISPs, because it burns so very, very much traffic. Many of them would like nothing more than for Netflix to die. If net neutrality goes away, they may start trying to intentionally throttle it, to drive you away, either because that will just get you to use less of the data you supposedly paid for, or even to help kneecap a competitor (Comcast, at least, owns a streaming video service). Using a VPN will go around the problem, at least temporarily... but if it's being done deliberately, they may start throttling ALL VPN traffic, too.
If you have a good provider, going through a VPN should give you worse performance, overall. But the lousier an ISP you have, the more likely it becomes that using a VPN will improve high-bandwidth, popular destinations, like Netflix.
(Now, if all their links are over-congested, or least both the ones to Netflix and to the VPN provider are choked, then you may not see a damn bit of difference. The only way to know for sure is to try.)