Is it too late to add more water?

I'm working on my second batch, a modified Fuller's ESB clone.

Partial Mash: 8oz Victory

7lbs Light DME
1oz Northern Brewer plug (Bittering)
.25oz Perle (Bittering)
.75oz Perle (Flavor)
1oz EKG (Flavor)
.5oz EKG (Aroma)
Wyeast London Ale Yeast (Liquid)

I did a 3 gallon partial boil, cooled, and racked to my primary, where I THOUGHT I filled it up to 5 gallons. I then pitched and let it sit. I took a look today and it's fermenting away, but when I looked at the volume, it was only about 4.33 gallons!

The gravity was higher than expected (1.075) and this is probably why.

My question is: is it too late to add water to the primary to bring it up to 5 gallons, or would that be a good idea?

I was thinking about taking the airlock out and siphoning in some clean water through the bung?

I'd leave it until it finished fermenting and if you want you could dilute it after it finishes with boiled cooled water to reach your desired alc/vol

You could but you don't want to introduce any bugs or oxygen if you can help it. That would mean boiling and cooling water without any splashing then racking splash free to the fermenter. You may be better off just letting it be. I would rather drink a strong beer that's a little malty than an oxidized or sour beer.

Thanks for the advice. I guess, logically, if my yeast is healthy enough I'll just come out with a stronger brew, which (as you said) is not at all a bad thing. Thanks for the advice!

You could add extra water at bottling.