First of all, I honestly want to tell you something: I hate Shiftry. But I will still help, no worries :)
If I understand well your dilemma, you have a Shiftry you evolved at level 45 for completing the Pokédex and you want it to learn Extrasensory, which is now impossible ? And you are wondering about catching another Seedot and train it until it become a level 49 Nuzleaf, thus evolving it with Extrasensory. No ?
I recommend you, actually, to catch another Seedot, train it to a level 49 Nuleaf and evolve it.
Why, you ask ?
Simple: Extrasensory is great on Shiftry. After all, if you train until level 100, you don't need 310 HP, as your speed and attacks are they most important. In-game, you don't need 345 HP, 300 Atk and so-on marvelous stats. Just higher than 265, this is enough. If you are worried about it so much, then train your level 45 Shiftry but seriously, get another one with these moves:
~ Giga Drain (Recovery, STAB)
~ Faint Attack (STAB)
~ Rollout (Tutor, coverage)
~ Extrasensory (Coverage)
And you are done.