We haven't actually seen the spirits in a legit battle to say one way or another really. We seen a little bit more with Gemini and Scorpio Sandbuster in Alveraz is stronger than in other arcs we have seen it. These are the only two times we have seen the spirits actually truly use magic to attack really. However based off Stardress which supposedly only use a portion of the Spirits power then it is clear they are much more powerful than they once were.
However power scaling is not consistent all around not just concerning Lucy. I mean the same Sword Erza used to easily slice through an Mega enchanted Meteor was easily destroyed by Kyria's blades and the same dude that burned through Infinite Magic and Time/Space can't seem to use fire in water.
Also I don't think Hiro mashima forgot about because not to long ago is makes a reference about how Lucy Stardress Mix doesn't really have the same power as Leo and Virgo in a way that made it seem weaker. Then proceeds to continue to say that Stardress only has a portion of the spirits power. He clearly wants to show Lucy off rather than her spirits but still acknowledge the power of the spirits through the stardress. It is hard to progress both without making her more powerful than you want her to be. For example if he wanted her to be equal to Gray he wouldn't make a Spirit equal to Gray because then when you factor in the fact she can summon multiple spirits at once and use Stardress then it makes her more powerful than Gray. So he tries to find a balance by making her combined strength of Stardress+Spirits equal to Gray. Mind you this is just an example not me saying She is equal to Gray so need for a debate. Back to my point though, Hiro simply doesn't really do it well though. With so many characters to focus on in an arc he doesn't have time to really give much thought to the balance so one ends up taking the lead over the other, Usually the stardress. However like I pointed out he uses the stardres as a way to still acknowledge the spirits powers.