Regarding the OP, I really doubt that creating open only power play would suddenly turn it into some pvp dream, or pvp nightmare, depending on your point of view. People looking for pvp are already in open and if you are flying around in open now, it is not like it is wall-to-wall players. It is pretty much a wasteland. I rarely see other players in open and even if I go to a power's system, you may see one or two other players. Rarely more than that. Where is all of the pvp carnage? Oh, never mind, it is in Deciat.
However, people are able to affect pp in solo mode with impunity, which really is not correct, imho.
You're quite right, I don't think the Open experience would change much, if at all.
The primary difference would be behind the scenes. Players who don't enjoy being in Open would just...stop playing. The less people involved in powerplay, no matter where they're active, the less interesting it becomes to other players. This causes a butterfly effect, as the players who were sticking around in some part due to the efforts of solo players leave, the pvp players have less things to shoot at, causing them to leave as well, eventually leading to a complete collapse.
Oh, sure, there would always be a few people pledged, hanging around, poking the system here and there for a bit, but make no mistake, twitching or not, the body would still be dead.