I don't know of anything that is anywhere near that shallow and forget about 100w. Is it the in wall depth that is 1.5" or is there a 1.5"deep box that is giving you trouble?
1) If it is the box it might be possible to hack out the back, especially if it is plastic (doubtful if it's a 1.5" deep box, most are metal).
2) Is it a block, brick or concrete wall furred out then sheetrock? Blast out the concrete or brick with a hammer drill and chisel.
3) You could relocate to one side with a LV ring then blank off the existing if you have the wall depth.
4) If none of these are possible your only choice is to use a Wiremold, Panduit or other box as an extender attached to the existing box. Maybe not the most elegant solution but not too bad.
-Hal
