1. It's pretty slow (1999 smartphone as Vitalik always says haha). Since block times are 12 seconds you could only do high frequency trading using something like lightning network, but vanilla eth no. Ethermint will have even faster block times iirc. So it's hard to do TPS estimates a la Bitcoin with ethereum because it's based on gas limits. But if you filled one block with only augur transactions (buying selling shares, etc.) you could fit about 300 tx in. Which gives you 25 transactions per second (this is without Zack's work and without any scalability measures for either eth or augur).
So already better than Bitcoin TPS, and we'll be improving as we implement more scalability measures.
2. Fees for transactions should be pennies (doing the math based off genesis sale that's how much they cost when testing). I would checkout blog.augur.net (there's a qualitative post I made there why we went with ethereum) and some posts by Jack on improvements to the consensus algo.
So you can use ether to pay for contract execution (but you don't hace to), you can pay with subcurrencies. You can make a market in a subcurrency, so you can use a stablecoin, sidechained Bitcoin, ether, whatever. Speaking of sidechains, I have an interesting idea on how to implement them without the mining concerns expressed on the community, will do a blog post on this soon!
3. So the main thing is scalability additions. There's also the aspect of I want this to be tested really really well. There were going to be some additional features between beta and release, but we're actually a bit ahead on backend work, so they'll probably be mostly all in place by the beta.