I have a 1TB 5400 HDD + 25GB SSD cache in the M.2 bay. The system is fast enough. Windows 8.1 boots in 15 seconds, even using a 5400rpm HDD. The SSD in the cache helps programs to start quicker, and may be the booting of the OS is slightly faster. Of course, if you use a SSD and install there everything, you will see a huge difference.
IT ALL DEPENDS how you use your laptop. If you say "your HDD is bottlenecking your system" I'd say that's not true. When I'm applying an effect to an audio track, the CPU is the one that has to do all the work. When I am rendering a video, the HDD has nothing to do. So, you may be wanna upgrade your CPU cause your everyday tasks use CPU.
In my case, I do this kind of things:
1. Unrar .rar files almost everyday
2. Apply effects to a song
3. Export and convert audio tracks
4. Compress files into 1 .rar
5. Extract audio tracks from videos
6. Playing games
7. Render videos
8. Use Photoshop
9. Merge subtitles, audio tracks and video tracks
And a few little more things. Well, for points 2, 3, 4, 6, 7 and 8, an i7 is what I need. For the rest of the points, the HDD is the device that is slowing down the whole process. Though there are some points that use both of them and if you upgrade any of the 2 components, you'll see a difference.
I have an i7, but I wanna replace my HDD by a 750GB SSD so I can use it for everything. When I'm extracting audio tracks from videos, or adding large audio tracks to a program, the HDD really slows down the process. When you enter to a folder, all the thumbnails will instantly be loaded if you're using a SSD. Every single program will load ridiculously fast. If I had to choose between an i5 and a SSD or i7 and a HDD, I'd to the the first option because you will notice the SSD in everything, and the CPU only in certain situations. AND, moving from an i5 to an i7 will suppose an improvement in terms of percentage MUCH MORE low than moving from a HDD to an SSD.