I put back the stock motor and bought new undrilled 9x6 propeller and went for a test flight. The vibration are much less pronounced, almost unnoticeable for the most part. It was rather windy, so the video was not as stable as the previous one. I made some high ascents (I assume the highest being to some 300 m). Apparently, there was some slope lift at the place, because my first glide (from the lowest height!) lasted almost 10 minutes (with heavy 5000 mAh battery and a GoPro in original housing), which is pretty impressive. At those distances I'm getting frequent RSSI beeps from my FrSky transmitter, wonder if the default setting is too conservative. I made total of three ascents. The last glide had to be aborted because of approaching air ambulance helicopter -- the abort with flaps and nose down worked nicely and I was on the ground in about 30 seconds (from less than 100 m, though).

I have replaced the stock motor with Turnigy D3536/5 1450 KV outrunner. I had to redrill the propeller, since the new motor has wider shaft. When I finally got to flight test it, it performed well, but something (probably the hole drilling) is not right, since there is quite strong vibrations. With the stock motor there were no noticeable vibration even over 50% throttle, but with the new one, most of the powered flight gives useless video. The flight as such, however, is nicely stable, as you can see from the video containing mainly the gliding sections of the test flight. Note: the video is just a test, it's long and has no sound.

Super Sky Surfer - Test camera flight (9 min 12 sec)