There's a lot of good general security practice information in the other answers, but as I've just been researching for myself the implementation of security in the HomePlug AV spec (used by the devices you mentioned, and many others), I thought I'd add a little more specific info not covered yet.
First, there's no such thing as unencrypted data transfer under the HomePlug AV spec (with some non-security-related exceptions). All data transmission is secured with AES-128. (Reference) What the encryption/securing/pairing process does is set up a new network key (referred to in the spec as the NMK, Network Membership Key) to replace the default key that the devices ship with. In other words, out of the box, your communications are encrypted, but not private (since any other device would be able to connect using the same default key). So it seems odd to me that you see a performance difference before and after going through the devices' "encryption" process.
With that in mind, the biggest security concern in a powerline network seems to be how to have two devices agree on a NMK, and how to distribute it to new stations joining the network. The spec supports a few methods for doing so, but ultimately leaves the implementation up to the manufacturers. The common one-button pairing scheme in a lot of the consumer powerline networking equipment seems to be consistent with a procedure that uses a protocol called UKE to transfer the NMK. UKE in itself is not secure. However, the properties of the physical layer of the HomePlug AV spec make it difficult for one station to eavesdrop on communications between two other stations. (Reference)
Some devices come with a utility program you can run to configure some advanced properties, which may include manually setting a Network Password (NPW). This is like a Wifi password... it is hashed to produce a NMK. If you have this option, and can manually set the NPW on each of your devices (and set a suitably strong NPW), then
I would consider that a secure system. (see update below) If you can't, then you have to decide just how likely it is that someone with the proper equipment and knowledge will be close enough to be able to listen in on the key exchange when you pair a new device to the network. If you're really paranoid, you might be able to try to do the pairing over some isolated power network (like a generator or an inverter in your car) and then move the devices into the main network.
UPDATE: Scratch that. Looks like there's a demonstrated weakness in one of the keys used in HomePlug AV. It's not actually a flaw in the standard itself, but in one method that a lot of the vendors use to implement it. So not all devices are affected, but the list looks pretty big. (Reference)