Im currently using NRF21l01 (32 byte) however I need more bandwidth any suggestions?
Surely nRF5x chips might give you bigger bandwidth but it depends on so many things... what exact protocols you use or want to use? What is untended application, use case, power consumption? Finally why nRF51 when you could go for almost the same BOM cost with nRF52 which is more powerful and should have lower power consumption in most applications?
Take a look at the infocenter page Bluetooth low energy data throughput