I think you get it in a good price and as I've said I believe it will have honest hashrate per watt efficiency.
If I remember correctly I saw a thread that an R9 290 running blakecoin (same algo) got between 2.9 and 3.1. Now if that translates over to decred is yet to be determined.
Decred has higher input size and more rounds. Your estimates are way too optimistic.From what I can tell one 290 will not mine anywhere close to 20k coins in the first year. There is no way that the total hashrate will be only something like a couple hundred 290s worth. Add at least one order of magnitude since there are people with large GPU farms that went out of business in early 2014 and will now reactivate. At least those who didn't sell out meanwhile.
5 days prior launch of mainnet. I heard someweher that a miner for testet was available 1 week before launch. Is this true?
Jan 18th signifies the close of airdrop sign up. The last tentative date for mainnet launch was Jan 25th, which is balanced against testing the software internally until it's ready for release. What is definitive is that testnet and miner binaries will be released one week prior to the launch of mainnet, accompanied by miner specifications - so that everyone can get comfortable and set up their gear. So the closer we get to tentative dates, balanced against those variables, the clearer the schedule will become (since we can't sacrifice stability and testing in favour of a release). However, everyone wants to avoid delays if at all possible.
Well comparing this preliminary results with the hashrate for blakecoin we can find here, I expect that we'll get about half the hashrate of blakecoin in Decred mining but it's just a speculation.