For me, after 10 years of driving my S 987.1  the best two (most noticeable) mods were:
In 
chronological order 
1) Adjustable coilovers (compression/rebound)
2) OS Giken LSD
The coilovers are amazing as you can lower the car without getting stiffer ride for the street and the LSD transforms the car completely.
Now that I no longer have the 987.1 these are my goals for the 981, so all I need to is start eating ramen noodles every day and start saving.. 
PS: The 3.8 LN Nickies & pistons were next on the 987.1 list, but somehow my destiny changed..