Let's clear this up (again!).

There are 2 routes, both of which get you 0.5 BA Miles for every 1 MR point. These are Starwood Preferred Guest and InterCon's Priority Club.

Each has an added perk:
Go via SPG and, if you transfer 40,000 MR points (=20k SPG), Starwood will give you a bonus 5,000 BA Miles. However, you don't have enough for this, so your 25k Amex = 12.5k SPG = 12.5 BA.

Go via Priority Club and your points (50k PC, which gets you 12.5k BA), count towards PC status. You will be almost at Platinum with your transfer.

However, at the risk of being lynched, you are better off putting your miles into BMI Diamond Club. 25k Amex = 25k BMI. You know this as you are a member looking at your profile, but for newbies: You only need 37,500 BMI for a 'cash and miles' business class ticket to the US, Africa, Carribbean etc (using Star Alliance), or 30k for business class to the Middle East. Since you can get a further 20,000 BMI miles by taking out the free BMI credit card, you could have a 45k balance in BMI within weeks, compared to 12.5k with BA.

Loads more on the BMI board about 'cash and miles' rewards.