You can do fine with competition color and no base tan underneath.
You might need a little more color, like an extra coat, but I don't think that having a base tan makes that much of a difference, esp if you go with the spray tans they offer at the shows now. It depends mostly on how well your skin absorbs the color, which doesn't change if you have a base tan.
Jan Tana's "On Stage" is VERY dark if you put relatively heavy coats on. I believe that's what she sprays.
I actually prefer Pro Tan now that they are offering Pro Tan Spray at the bigger shows.
Either way, though, the spray tan is the way to go. It's so easy and it goes on really nice.