First, consider this; can you spin out the 50t? If not, there isn't anything to be gained by going to a larger chain-ring. At a cadence of 90rpm and a 50t x 11t will yield over 32mph & a speed of about 42.5mph could be attained with 120rpm. Even with 50t x 13t & 90rpm, the speed would be close to 28mph.
If you make the switch, you may find the front derailleur can't handle the larger chain-ring, so check the specs first. Older Tiagra derailleurs could handle up to 53t, but the design was changed for better shifting, and the current model specification shows a top gear of 50t with a range of 20t (meaning the difference between the large chain-ring & the small ring can be no more than 20t, ie. with a 50t outer ring the inner ring could be no smaller than 30t)