It's not perfect tense... because teleioi is an adjective, not a verb in the perfect tense.
And yes, teleios can carry the idea of completeness, but that does not mean “mature but still sinful.” The context determines what kind of completeness Jesus is talking about.
Jesus says, “You shall be perfect, just as your Father in heaven is perfect.” God’s perfection is not simply that He is spiritually mature. He is morally complete and without sin.
The surrounding verses are also about moral conduct: loving enemies instead of only loving those who love you, doing good beyond the standard of ordinary sinners, and reflecting the character of the Father.
So I don’t think the verse can honestly be reduced to, “Just grow as much as you can spiritually.” Jesus gives God Himself as the standard.
If “perfect” merely meant “reach your personal best while remaining imperfect,” then Jesus could have said that. Instead He said, “as your Father in heaven is perfect.” Unless you think God needs to get more spiritually mature, which doesn't make sense.