I think as you play more, you will find that the prices are appropriately set. Price is not just based on power, it's also based on access.
One of the reasons Witch costs $5 is that it ensures equal access to everyone. If it cost $6, then on Turns 3-4, some people will randomly get $6 and then win. Since it costs $5, you can almost always get one by Turn 4.
I think this deserves to be restated, Dr Who. The price of a card is NOT measure of how powerful it is. It's simply how many coins it takes to get it. This is a compromise between many different things - when players are able to get it, how easy it is to pick up many of them, how likely it is for some players to get extra-lucky and get it early, and so on and so forth.
In addition to power levels:
$2 cards are ones that need to be accessible to everyone, even players who get a 5/2 start; they're also cards which it's okay for players to pick up in bulk with spare buys.
$3 cards are ones that it's okay to open with two copies of, if you start 3/4.
$4 cards are ones that mess up the game a little if you get too many of them immediately, but it's still okay to start with one of them on turn 1.
$5 cards are ones that you want to have a little bit of buildup before you get. You probably have to buy a silver or silver equivalent; or even two; you can't guarantee that you get those cards unless you deliberately aim to do so.
$6 cards are ones are late-game cards - it's impossible to get them on turns 1 or 2, and even fairly likely to miss them on turns 3 and 4. You may only get a few chances to get these before you have to start picking up Provinces, at least in a Big Money-like game.
And there are plenty of other considerations that I'm not thinking of at the moment.
So, when asking whether Witch should be moved up from 5 to 6, "would I take it above gold" isn't the right question to ask. There's plenty of cards that I'd take over Gold if they were at 6; and plenty of cards that I take over Gold while they're at 5, or sometimes even 4, or 3, or 2. (I might be a little hard-pressed to find a game log where I take a 2-cost card over a Gold, but I bet those situations happen occasionally.) The question you should ask is "would the game be better if this card were harder to get?" (Or easier to get, if you're talking about making them cheaper)
So, I would restate the suggestions to keep the prices where they are. They've been playtested a lot; I bet that Donald and his playtesting crew has tried basically all of the different things you're mentioning. Every single card you've mentioned has pretty good utility at their listed price. Instead of making them more usable, making them cheaper will likely turn them in to "must-buy" unavoidable cards, and making them more expensive will just delay when players can get them without making the game more interesting.