Grabpass vs HDR performance on Mobile

Greetings,
I'm using an overlay decal to brighten parts of the ground,
as LDR clamps values in a [0-1] range I can't do such thing with shader blending (DstColor/Zero)
So the only two workarounds I found were to either enable HDR or use a Grabpass in my shader to overcome this limit.
I'm already using post-processing, with that keeping in mind, is using R11G11B10 HDR heavier or using a Grab-Pass (post-processing is already using grab-pass, can I re-use the grabbed data)???
none of the above had a noticeable impact on the devices I tested but I'm concerned as some devices may have issues!
Can someone please briefly describes with one is better?
Thanks a lot.