My husband has that exact combination - DF508 and the 5T. We just found out, and I'm not sure the exact classification of the 5T. However, the only CF-symptom that he has (well, that we know about!) is CBAVD. In other words, he's lacking his vas deferens, which makes pregnancy impossible if not for IVF.
You may consider doing PGD anyway - especially since you're already doing IVF. That way, you can guarantee that the embyro doesn't carry either mutation.
For us, our child will always be a carrier (since my husband is passing down one of his two mutations). One of the things that I've thought of is that while our child may not have CF, they may very well have a child themselves someday with it (if their spouse is unknowingly a CF-carrier). So spending that money now may circumvent a lot of heartache and pain later on, too.
Also, if you have a boy, you'll have a 1 in 4 chance that they'll be in my husband's situation. Dealing with infertility - let alone a disease like CF, no matter how severe - can be pretty emotionally grueling, too.
Will your insurance cover it? Or do you have a good clinic that charges less than the typical $4-5,000?