I, heretically, went the other way and fitted a 17t front sprocket on my 2013 ST ABS recently and have been kicking myself for not checking the stock speed vs rpm before I did it !https://www.street-triple.co.uk/index.php/topic,34825.0.html
The Service Manual seems pretty clear about:
(i) non-ABS models having a "gearbox-mounted Road Speed Sensor" - and they, presumably, would indicate incorrect speed if you changed the number of teeth on a final drive sprocket.
(ii) ABS models not having the above RSS, and using the wheel-mounted ABS speed sensors instead - they should not be affected by sprocket changes.
According to the Gearing Commander site; for a stock ST with 16t, 5850rpm = 70mph in 6th gear
Your 15t should be; 5850rpm = 65.6mph in 6th gear
My 17t should be; 5850rpm = 74.3mph in 6th gear
I haven't checked but I will next time I go out.
And, I guess the above figures = actual speed rather than what the speedo claims. Probably best to use a GPS app to measure speed accurately if you want to check, but I think your (ABS) speedo is fine, and mine too.