Hi Reddy,
For K66 cache, need some clarification here. There are two different types of caches on chip. One is the flash cache, referred to as FMC module for flash acceleration so it can provide single cycle access time for instructions read from internal flash. The other is the local cache referred to as LMEM module which provides caching capability for external memories such as SDRAM. Please note on K66, only code cache is available, there is no system cache, so any address range above 0x20000000 is non-cacheable.

Default configuration on K66 is FMC cache is enabled, while LMEM cache is not. So if you try to run code on external SDRAM, you need to use the KSDK code to enable LMEM code cache.
For performance increase, you can try to run Coremark code to see the difference of enabling or disabling cache.
EEMBC - CoreMark - Processor Benchmark
Hao