I am inclined to say the suspension, as you will have better options on set-up - particularly if you include the adjustable ride-height link.
Having said that - the 848 has the heavier option cast wheels, so the BSTs will have a real impact in comparison.
748's laptimes are obviously counter to this, but the impact of the lighter wheels (remembering that I think 748 is comparing BSTs with light forged wheels) is arguably greater on uneven road surfaces, as opposed to the smooth surface of a racetrack.
So do the suspension first, and keep saving for the wheels. Against the wheels is the fact you have to be a bit careful who you trust fitting tyres to them..
And as to suggesting wilks get the wheels - of course he already has the wheels, SBK forks, TTX etc.
He rode it to my place one day (not far admittedly) with slick tyres on it. Really had it set up for the shops!