The RWR does not warn you in the case of an IRM being sent your way. It is, afterall, a Radar Warning Reciever, and an IRM is completely passive (it simply homes in on your heat signature). You are simply being painted by an enemy's radar, because he acquired you visually, but cannot get enough of a radar return off your ac to guide a SAHM or AHM.
Stealth is all or nothing in this series, unless its been fixed in the latest WoI patch., so setting the RCS value above a minus value will make you visible to every radar in-game, irregardless of your range from the unit. Setting it to a negative value will make you invisible to all radar, and it makes no difference if you decide to use -0.00000000001 or -0.9999999999 for a stated value.
Setting the "MaxVisibleDistance" to 2000-3000 will probably give you a somewhat more reasonable approximation of the real aircraft's low-observable characteristics, as the enemy AI will not spot you, nor will their controller, until you are inside of that that range (even at night). Admittedly, its a bit of a cheat, but even in RL, stealth technology is best employed in low-light conditions.