The Zefal fenders I put on my 520 are very effective. I also installed:
- Skewer Locks - effective so far (seat/wheels not stolen) but a bit of a pain when changing a tire due to poor ergonomic design of skewer wrench
- Vetta wireless computer - great so far
- M585 Pedals - I liked them for the wide support platform which seems to eliminate pressure points and you can ride the bike withour cycle shoes if you have to
- Arkel Panniers, no complaints yet, I chose the T42 side-load version
- Specialized BG saddle - saved my behind (literally) on my inaugural century last year

My personal best for speed with the 520 was 54.6 kph/34 mph(unloadedon, flat course, light tailwind) - not bad for a tank. I rode the MS150 last year fully loaded, no complaints.