As I mentioned... not all data is cut and dried. Without pressure listed in anyone's data (Speer, specifically, does not list pressure,) you don't know if they decided to test to max pressure, or just call it quits when they reached a velocity or accuracy node. I also mentioned... and it has been said umpteen times before even here at THR... one book's max is another book's starting load, etc, and that printed data is not the end-all in load development... but it is prudent to consider published data. I have a few loads that are over published max, I arrived at them using reasonable handloading techniques, and I assume all risk when using them. I suppose if I dug around on the internet,I could find published data somewhere to validate my load... but I'm not that gullible.
I don't think the OP is in trouble with his load of 2400. He's been loading it for years, the fact that he has raised the question tells me he is not careless. His shooting observations, looking for pressure signs, seems reasonable. BUT. He is the one assuming the risk. Personally, I would go to Sierra to see what data they have for their bullet, unfortunately, they do not post their data online. Comparing notes, Nosler's max for 2400, with their 240grn JHP, matches Speer's at 21grn. Hornady's 7th lists 21.2grn 2400 with their 240grn bullets.
My conclusion:
Comparing Speer's #11 to #14 (1987 vs 2007) and looking at all the data for .44MAG and 240grn bullets... ALL of the data has been changed, so it's pretty obvious they retested it. Speer's #14 data matches, or is very close to, two other reliable data sources. I circle back around to my comment above... it is prudent to consider published data.