I was previously running 2.8.11 on all of my routerboards, last week after some testing I decided to upload version 2.8.17. Everything worked very well but when I did a bandwidth test on the mikrotik's I found that all of my 2.4Ghz links were fine speed wise and that all of my 5.8Ghz links had dropped in throughput substantially. To give you and idea I have a 19Km 5.8Ghz link that used to report +-12Mbps when I ran the bandwidth monitor, after the upgrade I was battling to get 1800Kbps. My other 5.8Ghz link (+-20Km) used to have 16Mbps, after the upgrade this was also down to about 1800Kbps. I tried everything, Nstreme, frequency changing etc .. but to no avail. I then uploaded the wireless-legacy package and rebooted my Mikrotik and the speeds were back to normal.
I have subsequently been told that you need to first remove the wireless package and then install the wireless-legacy package, I however did not do this but have not yet experienced any problems so it's up to you.
Hope this is of help, give me a shout if it is not.