Let's be clear -- in the absence of a field approval (from a FSDO or DER), when an IA uses data which is not completely make/model/system-specific (like an STC) to the installation being done and signs a 337 based on that basis, the IA is putting his/her ticket on the line. If the FAA should later come to review that 337 and disagree with the IA's conclusion that the data were not both approved and applicable, the IA could be doing a rug-dance.
Not Really,
However, 337's using "approved data" in any form are, unlike requests for field approval, no longer reviewed at the FSDO, and merely sent straight to the repository at Oklahoma City, so the likelihood of such a review is not great unless something brings it to their attention (say, because the new installation catches fire in flight or something like that). In that sense, the FAA has placed great faith and trust in the IA's in the field to get it right, and most IA's I know take that seriously. As a result, if you can get an IA (or FAA-Certified Repair Station) to sign off the installation of that EHSI in your plane using the Sandel Installation Manual as "approved data," you're covered. The only remaining problem is finding an IA (or repair station) on your side of the pond who is familiar enough with this item and your plane to do that.