Hmmm...well, if your manual says not to do it for your specific model, then it would be best not to! I have a stealth, and the directions said any way was fine (or words to that effect).
On the subject of suction cups losing suction and letting things drop...oh, yeah. I have dealt with that! My best solution so far is to buy good quality suction cups, like Eheims (the black ones). And then secure them in place with tank safe silicone! Put a dab inside the cup, and press against dry glass. Then run a thick smear all around the edge, to seal it in place. I will use masking tape sometimes, to secure it while the silicone cures (4-5 days). The silicone shows up white when water is put in the tank, but a few plants to block the view usually does the job. Black tank safe silicone would be really great to find...