You likely end with a mess. We have tried to scale FH3 for larger drivers and we have not met with success. Personally i don't care much, since i feel the sweet spot for FRs is 4-5" -- larger drivers give up too much midrange finesse. If you want back what you lose to a larger driver, add a pair of powered woofers (note that i did not say subwoofers)
dave
If you really wanna go 8" FR in a BLH, you'll likely be looking at something like BIB, Ron Clarke's Dallas, or for really big scale Woden Vulcan - dual rear mouth.
And to echo Dave's post above, and other folks' sentiments, whether related to BLH types or not, 4"or so seems to be a really sweet spot for FR drivers - and very easy to add (multiple) woofers in compact enclosures for bass support - for music only, or HT