I think that when a boot is on the cusp between two binding sizes, there are always two questions: (1) Will it fit? and (2) What kind of fit do you prefer? If you don't know what you prefer, go with the smaller option and just max it out as needed. Personally, I've decided that I don't like the "crammed in the chassis" approach, even though I like tight straps.
I used to have Ride Spi's a few years back. I too was on the borderline of L/XL. I went with the XL, because the Larges I bought maxed out the strap adjustment for my boots. I returned them and got the XL's and cranked them down pretty tight. I was happy with my choice of size and binding! Loved the SPI's