@James, yes, the real test is to go live. We are playing a compounded return game, and as such we should always consider what it implies. You design a trading strategy and it has a modus operandi, a signature. It does not matter what it is, view it as simply designed to do this and that. From the stock selection to the actual trading rules all is specified except for the real gyrations of actual price movements. The trading strategy will react according to its programming. The strategy itself is what really matters and will make a difference.
You can view a trading strategy, in matrix notation, from start to finish as: \(\int_0^t (H \cdot \Delta P)\). It will operate from \(\,t=0\) to termination \(\,t=T\). A strategy could also be resumed as \(F_0 \cdot (1 + \bar g)^t\). If you add some delay you will be shifting the final outcome by \(\tau\) and produce something like: \(F_0 \cdot (1 + \bar g)^{t - \tau}\). It is the same as if you viewed the time horizon as: \(\int_0^T (H \cdot \Delta P) = \int_0^{t-\tau} (H \cdot \Delta P) + \int_{t-\tau}^T (H \cdot \Delta P)\).
What is being thrown away in an OOS or a walk-forward test is not the immediate future of a trading strategy, it is its future potential which can be valued at: \(F_0 \cdot (1 + \bar g)^{t - \tau} - F_0 \cdot (1 + \bar g)^t\). That number might appear trivial in a formula, so to give it more meaning, here it is with numbers: \(\$10,000,000 \cdot (1 + 0.20)^{25 - 2} - \$10,000,000 \cdot (1 + 0.20)^{25} = \$291,488,440 \). That is the cost of a 2-year delay. The cost of the delay calculated at the start rather than at the end is more modest: (\(\$4,400,000)\). Nevertheless, the real opportunity cost is the one at the end. It gets worse if you put in more years, a higher CAGR, or more cash on the table.
This would suggest going live as soon as you possibly can. And therefore, all the testing should be done prior to launch date. It would also suggest assuring all testing is properly done under similar market conditions as the trading strategy might encounter so that it would be ready to handle what will be thrown at it.
Naturally, if you do not design your trading strategy to last that long, then all this will not matter much since the strategy might simply go kaput. And in such a case, what would have been thrown away is: \(\$953,962,166\) the strategy's potential outcome. So, my suggestion is: do the best you can and as fast as you can. Delays are indeed costly.