I agree with the 7 kts but also bear in mind that you also have a higher thrust setting at the TH with F40 (especially at high gross weights). Combine this with a lower nose attitude and you can see that judging the flare is going to be more difficult. There is also a longer spool down time at F40 which in turn means the very real possibility of float (we all know this is an 800 trait!!). So your choice, take the extra 7 knots or just go for F30. Personally I think in poor viz it is the way to go, otherwise it is overkill and makes very little difference to improving landing performance....horses for courses
