Unless you are in a hurry my experiences says wait for the Canon 200-400f4L due out eminently. 100-400 is great affordable entry to long lens but IQ best at circa 8.0 and that is not so fast. The full range is not required, I find I'm between 300 and 400 most of the time at the track with this lens and mostly about 380mm or just off 400. 5.6 on the long side is not fast enough for motorsport,s unless the light or ISO/noise control in the body, is exceptional. IQ drops quick with TC.
This was my route to early motorsports imaging, 100-400 was my first L, specifically for Motorsports. Great reach and good images, recently picked up a 70-200 2.8 II, OMG!!What a difference a fast lens makes. Can only imagine 400 2.8 II Prime will be the one, but limited usability.
One fleeting thought if you are shooting local tracks with club events you can usually get close enough with nothing near 400mm required. At three sisters several places great with <200mm. Even Oulton or Donnington with faster shorter lens can give some great images. Foggy esses for example would be great with 70-200. For high end Pro events and the likes of Silverstone longer is better but not required. You may want to rent before you buy.