At least in theory, agents should not be able to stand toe-to-toe with the work. That's one of the premises of the setting- there's too much work, we're underequipped, understaffed, poorly managed, and the primary way to get through it all is bitter sarcasm and slowly collecting flamethrower parts.

In practice, it all comes down to how well you can write. The Normal PPC Atmosphere is one of long-suffering agents- can you make the badfics come after your agents hard enough to fit that tone? Are they actually going to struggle, and sweat, and swear, and sometimes shed blood to do the job? If so, they're not overpowered! If not, they could be ordinary humans and still be overpowered.

It's hard to write powerful agents. And that's why the guideline exists- it's not so much an in-verse restriction as an out-of-verse guideline to make missions interesting to read.

And if you go a single kilo-power above that, you'll be banned so hard, your children won't be allowed to join the Board, nor their children's children, nor their children's children.

But, uh, actually, though, I'd argue that there really isn't a distinct answer to 'how powerful is too powerful' beyond how said character's powers relates to the missions and stories you're telling. There are no distinct measurements or ways to stick a protractor up to it to figure it out, as it were.

The most important thing to remember is this:
Is there room for good storytelling, with these powers?

It is from this, I'd say, that the main issues of overpowered characters comes from: a character with ridiculous powers is very hard to write a good story with. There's not much of a story in a man who just solves all problems instantly, who just flicks away the most recent bland villain in an instant right after they've finished generically angsting.
It is also hard, too, to have a logical story with an overpowered character - your story might need to go somewhere very specific, and the powers bestowed upon your character simply do not work alongside that. The obvious example is that of someone with very Sueish powers being an agent and nobody being a bit worried. Your story wants your character to just be treated like any other agent, but with their powers, it's simply illogical for it to go that way!

It can be considered, then, not as a problem of where somebody lies on the 'power-level-scale' but a problem of good storytelling. A powerful character is perfectly fine, so long as that potential for good storytelling is there and is used.

There is certainly room for good storytelling in an agent whose powers cause a lot of worry Upstairs (and there has been, throughout various spinoffs).

So, yeah, that's how I'd suggest looking at it - thinking about things from a wider storytelling perspective, rather than being bogged down in deep-detailed-protractor-specifics. 'Too powerful' would be anything that inherently makes for a poor or illogical story.

Which, uh, is probably pretty vague advice, I realise. That's the sort of stuff that takes a lot of practice drabbling and beta-using to really gather.