Any metrics on how effective?
Unknown official sources and only personal stories to share, but since effectively the same idea was used in Iraq (used spaced armor-often times very thin) on HMMWVs to initially help defeat RPGs and curbside IEDs (not so effective vs buried IEDs). I can offer only conjecture it was overall relatively effective until better systems/vehicles were developed.
In 1969 in the A Shau Valley the CAV troop (and perhaps others) was sent out to be a decoy to lure an NVA Bn in the area into attacking us. A Bn of arty was to provide support to destroy the enemy by massed fires. All vehicles set up at about 1900 with wire as noted previously around each vehicle (approx 17 vehicles in all, M-48's & A-CAVs) in the troop. The NVA did attack at approx 2200 but the plan was a little off as it was not an NVA Bn but an NVA Regt in the area and the attack would last until about 0500 when fast movers were called in to assist and squash the attack for good. Not only did the NVA hit our troop with a full battalion but at the same time attacked the arty support bases thus nullifying most arty support throughout the night. The troop didn't lose a a single vehicle totally destroyed in the fight, though all vehicles were hit and damaged, but we were able to move 9 vehicles under their own power with minimal damage the next day. Though effectively overrun to the point of using beehive rounds to "dust off" each other throughout the night, no catastrophic vehicle losses were incurred while destroying approx 400 enemy KIA vs our 32 KIA and almost everyone else hit to some degree. Though some vehicles were hit & penetrated by BR40's/RPGs we found nearly 40+ rounds stuck in the wire or considered duds impacting vehicles. Though certainly not a perfect HEAT defeating system, in my estimation it was good enough!