So I'm slightly surprised not to see anything about this yet, so I thought I'd share the boss rewards I knew so far. There are prizes every 5 bosses, it seems.
5. Klout's Golf Irons (90/110) [melee]
10. Klout's Data Glasses (80/220) [armor]
15. City Escape Bike (300/200) [vehicle]
20. Klout's Gas Mask (220/580) [armor]
25. Klout's Targeting Pistol (700/500) [gun]
30. Corrupt Boss Knife (800/900) [melee]
35. Klout's Escape Grenades (600/1700) [explosive]
40. Corrupt Boss Rifle (1600/1400) [gun]
45. Klout's Surveillance Goggles (1400/2400) [armor]
50. Corrupt Boss Ride (2400/2300) [vehicle]
55. Klout's Surprise (3000/2700) [explosive]
60. Emergency Escape (2200/5800) [vehicle]
65. Dirty Business Hook (3300/3500) [melee]
70. Klout's Throat Cutter (5500/3800) [melee]
75. Klout's Shrapnel Spray (5000/5700) [explosive]
80. Klout's Shooter (6500/5700) [gun]
85. Klout's Sports Car (7200/6600) [vehicle]
90. Corrupt Boss Minigun (7000/8500) [gun]
95. Klout's Revenge (9000/8300) [explosive]
100. Klout's Super Car (60k/55k) [vehicle]
(credit belongs to the DFA member who did this, but I don't want to mention who-- I see you, top team vultures. Thanks bald zeemer for #90)
Additionally, the loot items receive a new set of rares every 25 levels, at which point the old rares become uncommon, and the old uncommons become common. (CK) EL detailed all of them here:
For what it's worth, the old L30 boss is about as strong as the new L50 boss, and the old L50 boss is about as strong as the new L70 boss. For Tier 5, anyway. I look forward to the insight of those who have completed the event to share just how brutal L75-L100 bosses are.
Also, very classy gesture on on GREE's part having "Klout on the Run II" start over from 0/300, even if your syndicate already blew well past 600. Thanks GREE. /s