Currently I have RAID configured on my PC in BIOS (not at software level). I wanted to use dual boot with ubuntu but this is not possible with RAID cnfiguration. Will my pc suffer in performance for gaming , computing etc. if i switch to AHCI from RAID ?
It depends on the type of RAID you have configured, but switching from RAID to AHCI you will either lose performance or redundancy. E.G: If you have RAID 0, you will lose performance on read and writes. If you have RAID 1, you will lose performance on reads, and lose redundancy. If you have RAID 5, you will lose performance on reads, writes, and redundancy.
If you don't want to lose the RAID performance, perhaps you could add an additional disk to your system for use with Ubuntu which isn't part of the RAID (if you have another SATA controller). Alternatively run Ubuntu in a VM stored on your RAID volume.
Additionally, if you change the SATA Mode from RAID to AHCI, you will lose whatever data is stored on your RAID volume.