I've noticed these JDM spinning reels on many different japanese tackle websites I'm wondering if any of you guys have used them and how they compare to the stradic/rarenium ci4+ and sustain. Thanks.

I've looked at many of these reels and there are many of them lol. The biomaster you referenced is basically s stradic fj. The main differences in the biomaster and many of the ci4 variants is shallow spools and a wider range of gear ratios. There might also be an extra bearing here or there. Most likely in the handle or under the spool. The two jdm shimanos that stand out to me from North American versions are the twinpower and vanquish they also command a premium price.