I don’t have clear answers to this question. As far as I know the answer does not exist. I’ve seen some stuff online if reference to the strength of Toyota joints but it seems that everyone on the forums is just repeating what they themselves read on a forum. Just because a bunch of people repeat something does not make it true. We have data on the strength of a 1310 series u-joint but we don’t have data on the strength of the stock Toyota. There are also a couple different sizes of Toyota joints. I’m finding sites that claim that the smaller Toyota joint is  tested to 3,700 foot pounds of torque before failure. Frankly, I think that is downright impossible. This small Toyota joint is physically about the same size as a 1310 series but the 1310 series joint is rated to 1,600 foot pounds minimum elastic limit, a 1350 series is rated to 2,200 foot pounds minimum elastic limit. There’s no way the small Toyota joint which is similar is size to a 1310 is more than twice the strength of a 1310 series joint and is 70% stronger than a 1350 series joint. In fact, 3,700 foot pounds would mean that this small Toyota joint is stronger than even a 1480 series Spicer joint. See the photo below for a size comparison. 

The problem with all of this though is that until there is specific and clear data made available on exactly how strong the Toyota joints are there is no way to make accurate comparisons between them and Spicer style joints. Furthermore, as mentioned earlier there are a couple sizes of Toyota joints. Below is a photo showing a side by side comparison of 1310 with small Toyota (right side) and 1350 vs large Toyota (left side).

You can see that the sizes are very similar, I kind of think of it as how a meter and a yard aren’t the same thing but they are basically the same thing. In this case instead of a metric vs American length measurement it is a metric vs American size universal joint. Anecdotally however I do believe the Toyota joints to be very strong and very good quality joints. I’d say that the smaller Toyota joint is about the same strength as a 1310 and the larger Toyota joint is about the same size as a 1350, based mostly on their physical size. That being said, if you do have the larger Toyota joint in your stock shaft and you switch to a 1310 series shaft you will be downgrading. If you currently have the small Toyota joint and switch to the 1310 you will be making a lateral change. Depending on the specifications of your vehicle I do think you may be right to be concerned about the strength of the 1310 series joint. If you want to get into the nitty gritty of which size is right for your application you can read more about that here. Or for general guidance you can refer to the following chart. 

I don’t think it is a surefire problem but if you push things hard enough it could break. If you do get a 1310 series shaft, and if you are pushing the limits of what that joint is capable of, I recommend getting it with non-greaseable joints as they are supposed to be stronger. Asking how much stronger echoes back to everything I said about the Toyota joints, we just don’t know. I have personally requested that information from Spicer and Neapco and neither company has provided an answer. They both claim that the non-greaseable joint is stronger but will not quantify how much stronger it really is.

We do have a 1350 series drive shaft for Toyota vehicles. We have teamed up with Trail Gear to develop some transfer case and pinion flanges, similar to their triple drilled flange, that will work with 1350 series components. These are only available for 27 spline transfer case output shafts and 27 or 29 spline pinion shafts. This option comes with it's own challenges though and we don't recommend it unless someone is running very large tires and/or a lot of added torque/power upstream from the drive shaft.