There are several reasons that polys have more spin potential than other stings.
- As others have mentioned, polys are less elastic, so with a spin-friendly swing, the strings bend downward (for topspin) and snap back in place, imparting more spin. The reality is that unless you are an advanced player, you likely are not generating enough racquethead speed for this to come into play.
- Polys are almost universally lower powered than other strings (due to the lower elasticity) so, even if you can't generate snapback, the lower power allows you to swing faster - thus generating more spin - without necessarily sending the ball past the baseline.
- Some Polys are textured or shaped and bite into the ball more than a round string, allowing you to generate more spin. Round polys seem to do this a bit more than other round strings, as well, though I'm less sure of this.
A poly at a high tension will have somewhat less power, somewhat less spin, less feel and ball pocketing, somewhat more control. A poly at a lower tension will have somewhat more power, more spin, more feel and ball pocketing, slightly or somewhat less control.
That same metric as far as tension works for all strings, synth guts and multis alike. Synth guts and multis have more power than any poly, so dropping the tension on them will result in noticeable increase in power.
I now play on softer polys, but played with multis for years. I haven't found a multi than generates nearly the spin of a poly, regardless of tension. However, a higher tension will not give you more spin on either a poly or multi. I have cracked the tension up to 59-60 lbs on a multi, it did not give more spin - though it did have good control.
I haven't played on a synth gut other than a demo since I was in high school and college (I wasn't a HS or college player, to be clear) so I can't say much about that. I did use a synth gut as a cross with a multi (RIP Control), but I really am not sure if that accomplished much - spin wise (or otherwise).
When I played with multis, I generally had them in the mid to upper 50's. I generally string my polys in the 40's - either upper or lower depending on the string.
Of the multis I played with, I found Head Velocity MLT to have pretty good spin for a multi and also a nice soft feel. Head Rip Control and Black Babolat Xcel were Ok. Lately, I've been playing with a Tourna Big Hitter Black 7 in a 17 gauge, and it's been a nice soft, comfy poly with excellent spin and control for me.