It seems that this suggested buff is a counter to the power of teleport/valkyrie, right? I don't think changing another unrelated card is the right solution though.
First, let's consider the thought of buffing cards, should it be done? I don't think so. In any card game there will be some good cards and some bad cards. It doesn't matter how much you buff the worst cards, the only thing that changes is that the worst cards are no longer the worst and now the second worst cards are now the new worst cards. Buffing just doesn't work in the long term. If nothing else it results in what's known as "power creep" where every card just gets more and more powerful until the game doesn't even resemble it's original form.
When there is a problematic card or combo of cards in a game there are typically three ways that they are dealt with, depending on the game.
1. Ban one or more of the overpowered cards (Magic does this)
2. Nerf the problematic card(s) (Hearthstone does this)
3. Print counters to this combo in a future set (L5R used to do this, a long time ago)
Note that none of these commonly used solutions involves buffing cards.
Now, let's consider the buff you suggested for a moment. Is Potted Powerhouse a weak card as it currently stands? I don't think so, I use it in at least one of my decks to very good effect. Next, would this proposed buff counter Port/Valk? No, it just gives plants a OTK like zombies have. One of the largest problems here is that Powerhouse and Bonus attacks are both in the same Plant Class (Mega-Grow). This gives 4 heroes access to the OTK. Port/Valk are two different classes and therefore only one hero has access to both. I think your suggested buff would only serve to deform the metagame much worse than Port/Valk currently does.
