I haven't seen much in the way of jacketed stuff for .38 going past 1200 or so. Maybe really light bullets like 85 gr, but a lot of the common stuff like jacketed 125's and 158 rarely make it over 1100.
9 I don't load because honestly its still relatively inexpensive to buy.
Using
Fed 155 gr syntec and
Fed 115 fmj as a side-by-side, there isn't a big difference in listed velocity.
I have seen some .357 hardcast data at around 1500, non gas checked. Might be in a lymans book.
The general rule of thumb is to just use any published load data like from
Hodgdon or anywhere else. I'm not saying its the be-all end-all for load data but its a good start.
You can get a little bit more out of coated because of less friction, and plated will go a little faster.
I don't necessarily think the issue is leading,although it is a factor because you can mix all kinds of lead together to get a specific hardness or coat it/ plate it. I think the issue is once you reach a certain velocity, anything past that and the bullet starts to pull itself apart due to centrifugal force. Soft metal can only spin so fast, I think the jacket keeps it from doing that.
Now I wouldn't try pushing straight lead, but even the mixed stuff with a brinnel hardness of 18 or better has a breaking point.
Gas checks I've only really seen for-
Carbine rifle data
Heavy/ compressed mag loads using a fast powder. - Even some of the lead-loads on the site list non-gas checked up to about 1500 or so with 180's
I have pushed some heavy stuff without gas checks. It might be powder dependent? Could be gr weight thats a factor?
A 180 gr .357 doesn't have the same amount of displacement and bearing surface than a 400 gr 45/70, so I'm sure theres some kind of "white man magic smoke-stick' math that determines what needs a gas check and what doesn't.
The above info may help or hinder.