My 4430 had a SmartTrax system on it with WAAS accuracy when I got it.  I struggled through half a season with it and started questioning the dealer about putting a Case/Trimble RTK system on it as I had all the stuff on my combine.  I was told the same thing you were by the local large dealer but the local Trimble dealer said they had machines steering down rows at 15+ mph.  I spent about $5k and made the change and it was the best thing I ever did to that sprayer.