It’s tiny. 3.91 is tiny.
Even the old Chevy 302 motor had a 4 inch bore.
You can fix stroke, you can’t do much with bore.
By your way of figuring, a 2.08 valve is 96% of a 2.15 valve.
Thankfully they used a 2.150 on the smallblock Victor heads, even though it’s only 4% bigger..lol