I don't believe you.
She gets 2.5 more mp5 from her base stats now and Spellthief's is giving her roughly 3 mp5 rather than the roughly 2 mp5 it did before pre-season. She gets about 7 more mana every 10 seconds than she did before, which translates to 42 more mana per minute. That's really rather good and means that after 2 minutes and 30 seconds of being in lane you've basically out-regened the mana pot you would have had, plus you get an extra health pot in its place.
Combine that with the buffed (and now worth actually purchasing rather early) Frostfang giving double the mana it once did and Frost Queen's Claim giving triple the mana it used to, your regen also scales exceedingly well into the latter parts of the game now.
Her mana will be fine. I realise Sona has her problems and should be looked at but mana is not and will not be the issue once the pre-season goes live. If anything I think season 6 is much kinder to Sona's mana issues than season 5 was, although that mostly speaks to just how horrendous mana regen was for mage supports this season.