Can - Buy cheap, buy twice... My choice would be Remus Revolution
Fork springs & fork oil, a must too IMHO... No need for progressives though
K&N Air Filter - Yes why not, its lifetime guaranteed (you don't replace it after every 8000 miles like a standard one; just clean it & re-oil it
)
Rear shock - Just fit a GSXR1000 standard shock, the one I got off eBay was excellent for £35
Cleaned it, fitted it and the difference in feel was massive