DBR: Silhouette, or WAP are definitely not the choice for light target loads. HS-6, Power Pistol, VV 3N37 or N350 wouldn't be either. Other powders mentioned like Unique/Universal or AA#5 can be used, but their not ideal when your talking about 148 gr. Wadcutters at 750 FPS. Since we're talking about +P, that's another matter.
I'm not sure I'm understanding your statements about .45 ACP, you said it works well, but later say it's not at its best in low pressure rounds like the .38 and .45 ACP. It is not intended for light loads, Period. While start charges will be lower pressure in .45 ACP, Silhouette is best used for higher velocity jacketed bullet rounds that operate closer to the maximum operating pressure of the cartridge to get higher velocity.
WAP followed the release of .40 S&W and was the powder used by Winchester to develop the round. The next cartridge Winchester used it for was 9 X 23mm Winchester. Powders well suited for factory JHP velocity in .40 S&W are also in the burn rate group you'd want to look at for +P .38 Special to get higher velocity levels at its maximum operating pressure, not as a do all powder for .38 Special.
Powders that are well suited to .38 SUPER and high velocity 9mm, fall into the same category in many cases. Examples: Power Pistol and HS-6 will get you +P velocity (not just pressure) in .38 Special and are well suited to .38 Super and 9mm also. Silhouette, or WAP, is in the same burn rate category as Power Pistol and HS-6. Slightly slower than Power Pistol and slightly faster but very near HS-6.
The fact that Winchester never developed data doesn't say much here. There are several revolver cartridges that Winchester only chose 231 and 296 for their load data. I don't believe that rules everything else out. Because of the popularity of the .38 Special, Winchester developed a bit more load data and one of the powders they've used (but not for Wadcutters) is 571, the equivelant of HS-7. Blue Dot has seen plenty of use +P loads in .38 Special and in older data shows the highest velocity potential for the cartridge, but you sure don't want to use it for light loads. 571, HS-7 and Blue Dot are fairly close in burn rate.
Another powder that Winchester overlooked in .38 Sp. +P is WSF that's faster than 571 and you'll find the same situation exists as far as data. Does that mean you can't use it for .38 Sp. +P, or does it simply mean that no one has bothered to produce data. Winchester most notably. Hodgdon probably won't now that they sell Winchester powders. As far as Ramshot data for Silhouette, all they did was reprint data Winchester had already generated for WAP.
One reason I mentioned Silhouette is because of its very low flash characteristics and its potential for velocity. Depending on your reason for developing +P .38 Special +P, Both of these characteristics can be very useful.