Ok guys - so at present that 150/60 rear tyre is just noticably smaller in overall circumferance than the stock size, almost 10mm. So obviously this has the effect of slightly lowering the final gear ratio. At the calculated rate it has the same effect on the final drive sprocket ratio of approx 1 tooth bigger on the rear. So basically just with the change in tyre size alone would give the equivalent ratio of 13/44. ( I know as i've just gone thru all the calculations for my WRR to go from the stock 120/80/18 rear to a 130/80 rear. This has the same difference in overall diameter as per your 17inch set up - only my change will be to a slightly taller tyre, so it will have the effect of gearing up to the equivelent of one tooth on the rear ).
Ok, so now add in the new smaller 12t front sprocket and with your current tyre size & standard rear 43t rear sprocket, overall this final gear ratio would be approx the same as a 13/47 set up. So quite a decent % in terms of a lower final drive ratio.
So in regards to the displayed speed with your current set up - basically it will be so far from reality that by looking at the speedo you will believe that you have the fastest 250cc 4s D/S in the world, lol.... Many say that the stock speedo is approx 7~10% too optimistic - but in my case my speedo in stock form was almost 15% too generous, doh ! ( All GPS tested & confirmed ).
In short there is no point in worrying about what the sprocket / tyre size changes will make to your speedo UNTIL, as others here have also advised, you fit a speedo correction unit. I have the 12 o'clock labs unit & can highly recommend it for price, quality, adjustability & ease of use.....
Have you had a few runs to peg WOT long enough to see what your best MAX displayed speed was ? If so please let us know what the magic number was, and we can then further tell you by how much your current speedo lies,
Cheers, Gavin.