For Atheros adapters, the best site to find good drivers is Atheros.cz
Unpack the drivers from my link in a folder and then update them manually in Device Manager... point the updater to search for the appropriate driver in that folder.
Check the connection speeds shown in Windows and in DD-WRT after you update the driver... looking forward to hearing from you !
After installing the new driver it now shows 150Mb on the laptop without the command lines entered in dd-wrt.
The old driver that was on the laptop was a year older then the newest one.
Great news ! So basically now your router can do 300 Mb/s and the WiFi adapter maxes out @ 150 (being a single stream device).
To conclude for anyone still confused:
For D-Link DIR-615 version D or H (Ralink chipset), the way to go is DD-WRT Firmware 23320 or 23503 (it seems that 23598 and 23720 can brick these routers), with 40 MHz + Startup command lines to enable 300 Mb/s. In addition, the latest WiFi adapter drivers should be installed to benefit from the fastest connection speeds.