to give you an idea i went from the stock 60R to a 50R and my speedo is out my about 5-7mph depending on how fast i am driving....with average speed of around 60mph my speedo reads close to 65mph. Now if i run her all the way out doing 103 mph the speedo reads about 120mph.....
Sear sells a set of compatible 17 inch rims for about 60-70 a pop. I'd install those, and then go low profile. For the tires, I am considering getting Continental ExtremeWeather DWS for myself (low profile, high performace all weather tires with erasable indicators on tires when it's not optimal any more drive on Dry, Wet, or Snow). They cost something like $100-$120 a piece.
If you don't want to change the rims, just get the low profile Continentals DWS for your 15 inch rims. The 15 inch ones should cost less. The wheels will look odd though, because Taurus wheel wells are too big even for stocks.
I run a 40 profile on my Mazda with 18's and I'm going to tell you, It's miserable to run lo-pros. Every harsh bump in the road, you are worrying about damaging your rims or worse. I dented a rim courtesy of ALDOT and their lack of putting up signs for a almost 3" change in pavement height at 65MPH. It also destroyed my shock and damaged my swaybar link. Also, Low pros on the factory rims would look bad without a suspension drop... and most lowering springs would still make the taurus too high (at least in the front) for lo-pros. Get bigger wheels then go lo-pro if you really want to. Just my 2 cents, I don't think A Taurus looks good on lo-pros without an extreme drop or some decent sized wheels.
225/55R16 will fit on the 16x6 factory rims, but it's a squeeze. I don't know if 225/50R16 will fit on the factory 16's, but they should fit on the 16x6.5" SHO rims. Plus, SHO rims look better imo. But those are just wider tires. You could go with a 205/55R16. They're common and look half-decently lo-pro.