hi! thanks again for your reply!
I'm fully sympathic with your hatred towards "marketing" numbers vs "real" numbers, this is indeed annoying and disgusting. However, what I'm currently trying to figure out is not at the ~10% level like we have with HDD, or wired LAN. I mean, in 100Mbit wired LAN I regularly get download speeds of 8MBytes/s, which makes like 64Mbit, right? Say, roughly, more than half of what was announced. Here, I bought 300Mbit/s router and a 300Mbit/s card. They do connect at an announced speed of 150Mbit/s - fine, let's accept announced MAX speed vs real. Now, when the "connection" speed is told to be 150Mbit, and the download speed drops do 16Mbit, this is 10x slower! AND it's ~20x slower than the announced maximum speed! Hell, they could as well sold it as 20Gbit/s router, who cares?
Concerning the restricted setup - yes, I've tryed switching off all the mobile and wireless phones, stick to N-only mode, and leave only these two single PCs running at home. I can't affect the neighbours routers, of course, but still, removing most of the interference is not helping much, the only thing is that my actual connection speed doesn't fluctuate like easter bunny from 0 to 3MBytes/s but stays roughly constantly at 2.5MBytes/s. That's the most I managed to get from 2 Dlink wireless-300-N devices (and another one is from Intel).
If no other solution is found, I plan to sell all of my external wifi cards on ebay and switch to wired ethernet-through-powerline solutions wherever possible. I'm fully fed up, and I don't buy those lovely fake numbers anymore. In fact, I would be glad a Dlink technician demonstrate me a setup where two PCs connect through my router wirelessly with half of the announced speed. I will then apologize and accept my foolishness...