Borat is not based on Mahir, although there was possibly some later influence.
The earliest date we can determine for Mahir Cagri's website is 1998. The earliest known mention of Sacha Cohen's Borat character is 1994.
Sacha Baron Cohen - Electronic News NetworkQUOTE
In 1994, the comedian sent to Channel 4 a tape picturing him in the character as Borat, a fictional television reporter from Kazakhstan, which caught the attention of one of the producers. Cohen came to prominence when his character Ali G started appearing on The Eleven O'Clock Show on Channel 4.
In 2000, in Da Ali G Show, Kristo re-emerged as Borat, the naive, racist, reporter from Kazakhstan.