What would be the impact on application startup speed in a system where the SSD only contains application data and the OS is located on a hard drive? Would I see difference versus the same setup, but where the OS is on the same SSD?
Is the OS files accessed when an application starts?
Many applications use windows APIs, they'd see a small effect. Certain programs would see huge benefits (see: anything that loads a lot of data from your hard drive), and some would see almost nothing (17kb self-contained exe will see huge benefits from no read-lag, but 4mb exe won't see much - it's not like your hdd is slow on that anyway)
since it is the operating system that is burdened with the workload of disk operations, it would be a waste to use a SSD for applications only, yes, the applications may load a tad faster but the overall performance will be nothing in comparison to a scenario where the OS is installed on the SSD.
modern operating systems are tasked with a multitude of operations in the background (multitasking :). so, the faster the disk where the OS is residing, the better the performance of the system in general.
SSDs really are good when it comes down to a machine under heavy I/O (Input/Output - Hard drive) load.
For example, the biggest improvement you will see is when you load Windows on a modern SSD.
Typically, it is hard to say the improvements you will see as it really depends on the application.
If you are talking about something like Photoshop, where it loads tons of external files and settings, you will probably see a huge improvement (at startup speed). However, if it is just a small, self contained application, it technically should be faster, however you are not going to notice as hard drives are fast anyway.
