I browsed your post and while I can't really estimate how well it works in reality, the way you presented your idea seems solid and encouraging.
One question I have: since the TUs used to determine reactions are lower by half the firing cost, it means they are effectively much lower. Doesn't it make reaction fire very slow?
Thanks – in my tests this worked very well, and balancing it should be easy. I think the only thing to keep in mind would be that a poor accuracy auto fire volley from a unit can be potentially very devastating to your own troops. So one would have to adjust one's game strategy accordingly.
The reaction stat is calculated using the formula: Reactions * (Current TU's / TU's). In my suggestion the change would be only Firing TU's / 2TU's, which is then subtracted from the Current TUs. That is not particularly much to begin with (since a typical shot is about 25% of TU's, the change in the reactions modifier is about 12,5%). But I also set 25% of TU's as a standard, meaning that in that case the modification is zero. (I wanted to avoid making all reaction stats lower: now the old reaction stat is simply the standard reaction stat.) What it means in practice is that for slow weapons the reaction score is 10% less for every 20% over the 25%, which is to say that the unit can move 10% further before the shot is taken.
Here are some examples of how it works (number is for the reaction stat) for someone with 50 reactions and 100 TUs, compared with the current formula that gives 50:
FORMULA: Reactions * (Current TUs - (WTU - 0.25*TUs)) / TUs, where WTU is the firing cost in TUs.
WTU 10: 54
WTU 20: 51.5
WTU 25: 50 (default case, i.e. as it is now)
WTU 30: 49
WTU 50: 44
WTU 75: 37.5
WTU 90: 34
WTU 100: 31.5
What this means is this: if the target has 50 reactions and 50/50 TUs, the reaction score is 50 (shot would be fired immediately on action). With WTU 50, compared to how it is now, the target would have to use 12% more of its TUs for the shot to be taken, i.e. 6. With WTU 75 that's 25%, i.e. 13 TUs. If the target has 100 reactions instead, as it is now the shot would be taken at 50% = 25 TUs. With the modified calculations, given WTU 50 the target needs to move 3 more TUs (6%, or 12% more of remaining TUs), given WTU 75, 7 more TUs. Notice, then, that the 10% for every 20% over 25% rule I mentioned above pertains to the remaining TUs: if the target's reaction score is much higher than the actor's, when reacting becomes possible to begin with, the change is even smaller. The main factor is, then, still the reaction stat.
In any case, the formula could well be tweaked for balance. The main thing for me is to allow for auto and aimed shot reactions, and the rest of the changes support making that feasible.
Thanks, and cheers!