I've turned the engine anti-ice ON many times at around 300-400' after T/O, seems OK to me, works well.
?? - you mean you did not crash? How do you know what your OEI performance would have been?
Well, we need JT or a similar wise-head here, but the reason, as I understand it, is that unless there is no loss of thrust with anti-ice on (ie 737-200 IIRC), then since your RTOW is predicated on obstacles up to the end of the acceleration segment, anything which degrades performance there has to be taken into acccount.
If it is not correct, I'll stop doing it!
Any performance experts around please?