I think you should first decide what your goal for the load is. Common goals are light load/low recoil, accuracy, hunting, anti-animal defense (not hunting) while walking in the woods, anti-personnel defense; the list is not exclusive.
For my money (and others will differ), lead slugs in a .38 are not useful for anti-personnel or hunting. By definition, you don't need a full power load for low recoil. And experience says that, at least for the .38 (true also for a lot of cartridges, but not all), accuracy is not a function of power. As noted, the 2.7 gr. Bullseye load has been accepted as superbly accurate (while having comparatively little recoil) with either the 148 or 158 gr. slug for years.
For what it is worth, the 158 gr. round nose slug at the power level you're looking at was the standard uniformed police load for 40 or 50 years; and it was considered just about useless for any purpose. For routine practice (both National Gallery style target shooting and old-PPC-style combat practice), l like the Keith-shapes (basically a truncated cone or mild ogive "semi-wadcutter") because it is intrisically as accurate as the wadcutter and faster to reload with speedloaders. If I were going in the woods where I was concerned about an unwanted encouter with a critter, I wouldn't have a .38 (though I might have a 158-gr. jacketed load in a .357), and for anti-personnel defense, if I were carrying either a .38 or a .357, my favorite load is the Remington 95-gr serrated jacketed hollow point. (Just as likely, though, I'd carry either a 9mm or .45 auto for this purpose.)
(Two points: I do not regard reloads as acceptable for carry loads, at least every day carry loads, for a bunch of reasons, one of which is that they are less resistant to degradation due to moisture and oil migration. Second, while energy (either at the muzzle or at the target) is a function of the square of velocity, recoil is a function of the first order of velocity. As a general rule, therefore, for a given energy level, the lighter slug will produce it with less recoil. The 95-gr. Remington comes out of a 3-inch barrel at better than 1,000 fps, but has what feels like less than half the recoil of a factory 158-gr .357; less recoil means a quicker and better second shot.)
So (finally) back to your question: would I push your 158-gr lead slug load closer to the max? No. In part, as noted, I think fast and lead don't go together, but putting that aside, I don't see the point.