There is no published data that will allow a 44 spl to be loaded to the same performance level of a 44 mag using the same bullet. That statement is blatantly false. The 44 mag moves the same bullet faster. Hornady doesn't even recommend the use of the same powder for their 240 grain bullet. If you want to use Accurate powder they recommend #5 for the Spl and #9 for the Mag. The difference is the burn rate. #5 is a much faster powder designed for lower pressure cartridges like the 44 Spl. The differences are night and day just like the 38 and 357.
We've done this dance before. Read your post again, you said "it's hard to load the .44Spl up". THAT is what is patently false. No one claimed the .44Spl can be loaded to TRUE .44Mag levels.
There is absolutely published data for handloading the .44Spl beyond SAAMI specs. It may not be in YOUR manuals and that may be YOUR requirement but the data is there, it's pressure tested and we know what guns it is safe in. Not only do we have Keith's and Skeeter's famous loads but a whole set of loading data from Brian Pearce. If it's printed in Handloader magazine and therefore added to LoadData.com, that is published data. I don't need to have pressure testing equipment, thank you very much, because I am not developing new loads. I'm using published loads. No one is flying blind here.
I don't own either a 44 Spl or a 44 Mag.
Shocking. It's always those who have never done it that seem to be the experts.
If you want 44 mag performance why don't you just buy a 44 mag? A few more ounces of weight isn't going to slow most people down and most would probably like the extra weight if they shoot 240 at 1300-1400 much.
As I've already said, 1200fps is not a .44mag load, unless the bullet is 355gr. A 250gr at 1200fps is a heavy .44Spl load and there are plenty of guns for which it is applicable. A .44Mag load in that velocity range is going to be way under maximum pressure. So yeah, your manual may have 1000fps loads in it but that doesn't make them true .44Mag loads. The heaviest .44Spl loads are about 26,000psi, the .44Mag is 36,000psi. That's a significant difference.
I have a bunch of .44Mag sixguns but that's irrelevant. I also have a bunch of .44Spl's but they are different guns for different purposes. Not even all my .44Mag's are created equal. Some, such as the N-frames, are really just slightly stronger .44Spl's. Others, like my custom Bisley and Super Redhawk, are heavy duty hunting guns capable of 50,000psi loads. Different guns for different purposes. A svelte .44Spl may not be your thing but it is to a lot of others and has been for a lot of years. Here's an example, a custom Old Model Ruger .44Spl that is 37oz. Or about a half pound lighter than a comparable .44Mag. Below that, a trio of GP's that range from 34-38oz. What may be 10oz or more less than a Redhawk. Below that, if SAA's are your thing, they are definitely more svelte than any Ruger .44Mag. So tell me, why would I carry a much heavier .44Mag, capable of a 355gr at 1200fps, if all I need is a 250gr at 950-1200fps?
The truth is not many people make them anymore because the 44 mag put the cartridge on the endangered species list.
Again, ignorance is surely bliss. There have been more .44Spl's offered by various manufacturers in the last 10yrs than at any other time in history. Including Ruger, S&W, Colt, Uberti, Freedom Arms, Charter Arms, USFA and probably some I'm forgetting. So no, it is not on the endangered species list. It's only those who don't understand who think the .44Mag should obsolete the .44Spl and that is sad.