This can vary depending on many factors… if a node had free space while ot a lot of test data was being heavily sent out for future tests, it could earn more during the test phase, for instance.
Also, the type of disk, network quality, machine power… Many things can impact nodes’ performances.
The estimators is just an estimator… But in average, it proved to be quite accurate.
Besides its author is regularly updating it to reflect past months observed performances.
In theory I guess, if the vpn provider allows for port forwarding… But that would add another point of possible failure to your setup, and additional latency.
This said, I’m really not used to VPNs, someone with knowledge on that should hop in here to give you better insights on that matter ^^
Also note that the vpn provider would cost some money too, so one should carefully estimate if it’s worth it or not. Especially as one vpn per node would be needed…