well i build AP lux very squishy(i also like to play support lux which is different story)...which is the normal thing to do...i jsut expect to use my range and my other teamates to protect me. I dont get RoA or Rylais...the only 'tanky' item i might get with AP lux very late is the hourglass and thats situational on if the enemy team has strong gap closers...anyway that makes you more of a presence in teamfights, but the downside can be in the lanign phase.
Ive never played ahri and havent been mid against her often so maybe thats a problem, i dunno
I personally always found lux to be far stronger then ahri. A skilled lux can be devistating to a team, that laser roam....