I'll add that when it comes to action photography you often need to use higher ISOs. In your first horse shot you've used what I consider to be a good shutter speed as it has frozen the details and the motion - however the light is not enough so you've got two further options to improve the exposure:
Wider aperture - smaller f number (which you've gathered from this thread already)
Higher ISO - get the exposure right and you can get pretty high with this, yes it comes at a cost of noise, but you can control and reduce that with a proper exposure and with editing - the blur from motion though you can't fix.
You used ISO 400 in the horse shot - so I'd have say ISO 800 or 1600 would have been better (and on the 7D 1600 should be very usable with a good exposure).
In addition with regard to the 70-200mm comments and teleconverters. First up I agree that idealy you need/want the f2.8 versions for the additional light gathering. This helps not only improve your shutter speeds when in lower light conditions, but also helps your AF (and the 7D has a cracking AF once you tame it through custom functions) since there is more light to gather and supply to the AF sensors.
Now you can put a 1.4TC on any of the 70-200mm lenses (f4 or f2.8) and keep AF and also have very good image quality. Of course you lose one stop of light so f2.8 becomes f4 and f4 becomes f5.6 (so again its promoting the getting of the f2.8 lens) but you gain range up to 280mm at the long end.
When it comes to the 2*teleconverter that would take away two stops of light - so you'd need an f2.8 lens otherwise you lose auto focus (AF stops on any camera body that isn't 1D line once the lenses widest aperture is smaller than f5.6 - the 7D can however get past this in live view mode only, but its AF then is slower than normal). You'd only go for the 2*TC if you need more reach - then you'd have a 140-400mm capable lens. However its image quality takes a bigger hit than from the 1.4TC. In general the common view is that the 70-200mm f2.8 lenses can't take a 2*TC that well (some disagree and say its fine - but the majority don't like the setup). That is with the exception of the 70-200mm f2.8 IS L M2 (yeah the really really expensive one - sorry

) which can take the 2*TC and deliver a good image quality.