Has anyone done this ? what were your results ? How much of a vel increase could one expect using 150 grn bullets over regular ammo say WW power points ?

No!
Hornady states on the box that the Light Mag ammo is for fixed breech firearms only. This includes pump, bolt, and lever actions, not recoil or gas operated arms. This warning (mine)applies to use of Federal "High Energy" ammo also. The powders used in these cartridges is too slow to properly work the actions of semi autos and can cause damage.
The Hornady Custom ammo uses the same bullets and has given good accuracy in the few calibers I have shot, If you you want to use interlocks and don`t reload, I recommend trying them instead of the light mag in your BAR.

Hornady states their Light Magnums are not to be shot in semi-autos. Like wise Federal states their High Energy's should not be shot in semi-autos.
In simple terms NO.

Browning only reccommends use of standard loads in the BAR. The pressure curve for the light mag loads is different from standard loads and may damage the gas system.

If you need the extra velocity trade, or even better, just outright buy a new one in 7mm or 300 Win Mag. This sounds to me like the perfect reason to "need" a new gun.