Thanks for the replies.
Please see the attached page from the 1980 POH that shows the difference between the two wheel sets. On the left, the TAS with spats at 2000'/75% power in ISA is around 110 but on the right with fairings it's 120.
PA28 Cruise Speeds
I thought this might be solely due to the difference between spats and fairings but further research shows that there is also a prop difference on serial numbers of 789001 and up which I presume is a coarser pitch. If that's the case then whole exercise becomes too expensive to justify and trading up to a faster aircraft would seem the way to go. It would appear that it was the 78 model that commenced sales with fairings and a 62" pitch prop.
The other alternative is to spend a reasonable amount on gap seals and enhanced leg covers and get what I can out of the aerodynamics. The question there is whether the parts sold in the states on FAA STC's are able to be fitted here without a new EASA STC being raised? Any takers on that one?
Wrench: The corrosion settled down to be mostly on the inspection panels and elevator. The aircraft is booked in for repainting next month. Thanks for your help. It was deeply appreciated.