Just played a new alt today, after ~2 - 2.5h I was ready to start m+ at ilvl 225ish, played my own key at +2 up to +5 then +8, got a couple small upgrades, sitting at almost 230 now at exactly 4 hours played. Played my friend's +2 (who's sitting at 210 ilvl, so even worse). Created group "+2 new alts", and invited people around 225 - 250ish, so no carry runs. TBH for those low keys you don't need to be carried at all, I was top dps with my shit gear once, and almost top the other times, and we still easily beat the timer for +3... the real advantage when inviting a higher ilvl character for low keys is the guarantee they'll be able to trade if something drops.
I don't think getting to 250-260 within 10 hours played is as hard as I thought, especially if you get a vault in these 10 hours.
But I did send a shitload of Soul Ash and Cinders from my other characters, as well as ~3k gold for gear from the AH and another few k for consumables.