Looks like the fork you have is 402mm. Handling is subjective, but I think over 20mm increase you may see a difference. By 30-40mm it would handle quite differently.

A typical 80mm fork is going to be at least an 80mm increase. For example the Tock Shox XC 30 is 490mm in 80mm travel mode. That is almost 90mm increase.

Not only would that rasie the front end, but all the geo angles on the bike will be something like 5 degrees slacker. A slacker head (from 70 to 65) angle drastically changes the steering handeling, and with a 67 deg seat tube angle moving the seat post back, you may not be able to slide the saddle far enough forward. And you will raise the bb height (and this the height ypu ride at) at least an inch.

And all this in addition to the fact that this frame was not intended to run a long fork and do the type of riding that you are talking about. That frame does NOT appear to be a re-purposed MTB frame. The angles are all road bike.