Running Raid0 on MBP 2011 I have a higher end (i7, 2.5ghz, 16gb RAM) 2011 MBP with an older HDD that has been pretty unreliable for the last year or so. I have an unused/unopened 250bg SSD sitting on a shelf and was thinking of buying a matching one for ~$60 and a Tray Adapter ($8) to replace the optical drive and running both SSDs in a RAID0 config.

I do both design and front-end dev work so I typically have photoshop and illustrator open along with a bunch of browser windows and tabs and/or several VScode tabs while running a LAMP or MERN stack along with a bunch of browser windows and tabs. 

My desktop can handle all that at once, but if I could get away with running even half (either the PS/Illustrator/Sketch/XD side OR the VSCode/MERN) at a time on this laptop, I'd be psyched. I only use while on the road (which is not very often) and while making system or hardware changes to my desktop.

The main drive is SATA3 and the optical drive is SATA2, which I'm sure would slow things down at startup using RAID0. But I'm wondering (hoping) if, that aside, the RAID0 configuration will boost performance/speed for the kind of work I'll be dong? I'm sure anything will be an improvement from the HDD, but will RAID0 (with 1 disc being connected via SATA2) give me a noticeable increase in performance over just a singe SSD on SATA3? >= $68 worth of improved performance? =)
If you have to stripe data across both drives, then you'll be constantly limited by the SATA II bottleneck. 

You'd be much better off just buying a larger SSD for the hard drive slot, or installing two separate SSDs as separate drives.

At best, you might want to look at creating a Fusion drive between a larger mechanical in the Optical slot and an SSD in the HDD slot. (Or even two SSDs...?)