You have to account that transmitting takes time, depends on your packet length. You should do some trials and measure the time, a good way is to use a logic analyzer and flip some IOs before/after a transmission. If you use ACKs that's roundtrips and retries, adds a lot more overhead.
You can always sleep between transmissions. But if you sleep 2ms and spend 18ms transmitting then you probably should look elsewhere for saving power more significantly.
My guess is if you transmit a LOT, then your power needs are very high compared to what you can save by sleeping a few ms in between transmissions.