It really depends on what type of files you use.
If it is MP3, even the 530 is a huge improvement over the stock iem. I would not waste my money on anything more expensive.
If you use lossless files or WAVE files, the 535 will do very nicely. The 846 may sound a little better better, but in my opinion, you will not hear the full potential of this iem unless you get a better DP with better DAC.
With many older versions of the Astell & Kern on clearance sale soon, you can grab one around $500. The combination A&K and Shure 535 will sound better than the iPod Touch and Shure 846, you also get the advantage of playing HiRes files, which will,sound even better.
In short, if you want to keep the IPod Touch, the 846 is probably an overkill. I’d save the money and get the 535 at half the cost instead.
What about if you were using an iPhone 6 instead of the iPod touch. would the 846 with iPhone 6 sound better with lossless files vs. the 353 with an A&K (and same lossless files)?