Well, Raisinbran, I think you have some wrong hypothesis here:

First you are assuming that your signal win rate will keep constant in the future and I think this is a big mistake. Try to backtest your signal in different periods, i.e. from 2013-2015 and 2014-2016. Does it give you the same win ratio in both scenarios?

Second, you are not thinking on real execution issues. Even if your TP/SL orders are placed in a "equally" distance, at the end will be executed as market stop orders, so you should assume some slippage. Also, you are not counting trading costs: spread and brokerage commissions, and depending on the instruments you may have other additional charges. So in practise your theorical 50-50 R/R won't be symmetric.

There are many kind of "Martingales", some of them interesting depending on the system. I recommend you to study them and to understand the exponetial risk increase of each one.

Cheers,