Don't go throwing the Oil Cooler away...
The traditional Rx7 racer will race with the stock radiator and early Oil Cooler as they came from Mazda, it's plenty to keep things cool... BIG IF... If they are in excellent condition..
Cavetation by the water pump occurs over the stock red line limits of the Engine... So, yeah you may need to invest here, but only after you start revving the piss out of the Engine.
Ideally when you get the money, get the Rad and cooler professionally cleaned with a sonic cleaning device - this has been discussed on this web site at length and you should be able to locate some information about the process.
Now, if you run out West in the Desert or in 100 degree temps often, then look into the bigger radiators and Oil Coolers, the basic concept of a stacked cooler and Rad are sound though, and the concept is worth keeping if you upgrade.
The number one sin for an Rx7 racer is letting his "lump" over-heat.. Never, never over-heat your baby, it will be certain death for her. If she over-heats at any time - ANY TIME! - just slow the pace down, if it won't cool down quickly after this, pit immediately! or pull off - DO NOT try for that extra lap - you will cook your baby...
Other_K