I don't see the spirits power scaling in proportion to the owner. The owner spends a certain amount of MP keeping the gate open, and whatever is left is free to make the spirit stronger. Stronger keys also tend to take more MP to maintain, so it all basically balances out to each spirit getting more or less the same buffs. The difference between Aquarius and Loke isn't that large to begin with. So with Lucy making each of them stronger, the gap between them becomes less relevant. I think all of Lucy's spirits are pretty much on the same tier to a third party. The difference among themselves is slight.
Lucy thought Taurus was good enough to be an opponent for Elfman after seeing what happened to Loke. So even if Loke was stronger than Taurus overall, the difference can't be that huge. And he might not be stronger at all when it comes to physical strength.
I don't think all golden gates, even when summoned, are same tier either. Lucy considers Loke and Aquarius as her strongest spirits. Remember when she was wondering what to do when Aquarius bailed on her? Something along the lines of "ohh what do I do? My strongest spirit left. Wait, that's not true " and then summoned Loke.
Angel's MP >> Lucy's. Yet Loke managed to easily overwhelm Aries. Angel herself acknowledged that "Aries can't hold her own for long against the leader of the zodiacs". This also kinda proves that golden keys summons are around same level.
If we're looking at feats, Loke defeated Bixlow, Natsu/Gray tier character at the time; and Zoldeo, one of the 7 kins. He has by far had the best feats of any spirits. Managed to take down actually strong opponents. back when Lucy's MP wasn't fodder compared to rest of the team. If Lucy has respectable magic power, Loke would undoubtedly be a very strong force to deal with.
Taurus is the most pathetic golden key spirit. Not because he's a comedy relief, but because he's the lord of Ls. Has the guy EVER done anything remotely impressive? All he does is take massive fucking Ls lmao.
Lucy didn't think Taurus could defeat Elfman, she ran out of options and summoned him (cause what's the alternative), only for him to get one shotted immediately by Madmole.