There isn't really tiers in performance (well, there are, but its not the only ranking), its really split into local, national and international races. Its difficult to say which series are faster than others too, because the rules for many of them are constantly changing and "who is fastest?" largely depends on the type of track and rule set.
Racing series are really tiered by level of competition and the cost to race.
I am not well knowledged in the American racing scene but I know globally it goes something like:
Open wheel:
-Karting
-Formula Ford
-National Formula Renault/Formula BMW/Formula Palmer Audi/manufacturer spec series formula
-National Formula 3
-World Series by Renault/Formula 2/GP2
-Formula 1/Indycar
Sportscars:
-Karting/Fomrula Ford/National Formula/Smaller sportscars (like the Ginetta Juniors)
-One-make series Cup (e.g. Renault Clio Cup)
-National Touring Cars/National Endurance series (usually based on FIA GT or Touring rules)
-FIA GT
-Le Mans Series
There's a lot of cross over and there's some weird anomalies like WTCC which is really at the same level as national series like BTCC in terms of speed but the cost to race is greater (as its jumping around the world rather than in one country).
Its fairly common for most of the sportscar drivers to come up through open wheel categories and switch because there is very little support at the lowest levels for sportscar racing, at least professionally anyway, you could graduate from historic touring car racing or various other local meetings.
Also, I've based this ranking on the European racing scene, its probably a little different in the US as I don't believe the same kind of support is given at least in open wheel.
From what I've read, its more tuned around leading to NASCAR rather than anything else although there is plenty of local level racing going on.
I haven't included NASCAR as I don't know enough to be able to rank it in terms of speed, costs and publicity compared to other series, I'd actually consider it a seperate ladder entirely. Its not really a sportscar series in the general sense.
But the top of the NASCAR ladder is easily up there with F1, Indycar, WRC and the Le Mans series for publicity and level of skill. The Le Mans series is the least publicised top line motorsport.
I'd rank SuperGT at the same level as the Le Mans series, which leads into my final point - the sportscars ladder may change soon because of the ACO World Sportscar Championship, we shall see if it returns to past glories. If it is successful, SuperGT, LMS, ALMS, etc will be at a similar level.
But if I had to rank them by performance on a circuit, my guess would be:
1. Formula 1
2. Le Mans prototypes
3. Indycars
4. NASCAR/FIA GT/SuperGT (not sure, don't know the laptime comparisons)
Formula 1 will always be fastest on circuits because of the huge levels of downforce and high power outputs. Le Mans prototypes are designed for maximum speed on the Mulsanne straight and endurance, they aren't special in the corners but have a greater amount of development than Indycar so I'm guessing they are faster. Indycar still has decent levels of downforce but not piles of horsepower, so its certainly ahead of the other sportscars.
The rest are probably very close, without looking at laptimes I can't guess.