I also emailed this same question to the guys at R2 Bike, which is who I ordered the Leonardi Spider from. I got an email back from them stating that the Q-Factor was 5mm less on the SiSL2 Crankarms. This would mean that each Crankarm is going to be about 2.5mm closer to the Chainstays.
I am currently still collecting parts for this build and have not begun assembling it yet, so i am not sure if that 2.5mm on each side is going to be a big deal or not. I want to use the SiSL2 Crankarms, because I believe I will save an additional 35grams or so over the SiSL Crankarms. I have yet to see a picture of a Carbon Scalpel 29 with SiSL2 Cranks on it.
I have a brand new set of SiSL Cranks already and the SiSl2 Crankarms should be here this week, so I will have all options available to me to try and see what fits.
Hopefully the reduced Q-Factor of the SiSL2's will not be an issue and will clear the Chainstays enough. I guess we will see.
i have a carbon scalpel 29 and a set of SiSl2 arms sitting next to me. unfortunately for you, i have not swapped the SL arms out in favor of the SiSL2s yet.
don't think i'll get to actual installation in the next day or so--but for sure I will in the next 2 weeks if you can wait.
i do measure ~11mm of clearance between a large scalpel frame's chainstays and 175mm SL crank arms on each side (a bit less on the DS if you choose to run the fabric chainstay protector). clearance would be GREATER than 11mm with arms shorter than 175. also, just by rough eyeball measures, the SiSL2 cranks *appear* to have the same Q factor.
if you're worried about 2.5mm per side, i'd say you will be fine. i do not anticipate any problems on my own installation.
the arms are cross-compatible with the spindles.