Talk about bad advice...

You want a mechanical system without problems? 7spd.

Or look for more cable pull per shift; AFAIK SRAM is better in this regard.

Actually, I kinda doubt friction is the whole problem here. I gather you have a full-length housing run inside the frame, pretty much a worst-case scenario friction-wise... but it's also a worst-case scenario for housing compression, which is a serious issue with normal cable runs above 8spd. You may benefit quite a bit from ultra-spendy truly compressionless segmented housing like Vertebrae...