For me, the question is less about the "sketchiness" of your own conduct and more about the ramifications.

 - They sound *dodgy.* **How sure are you that you're going to get paid at all**? If they're willing to screw your company over, how can you be certain they won't do the dirty on you as well and can you afford to sue them when their cheque bounces?

 - Are you breaching the ***letter*** of your contract and opening yourself up to legal consequences? Are you at risk of being fired for breaching NDA, anti-poaching or anti-moonlighting clauses?

 - Are you breaching the ***spirit*** of your contract and opening yourself up to bad faith and the negative consequences of being perceived as having taken business away from your company

 - If it goes horribly wrong, are they likely to want your company (and by extension, you) to work on future projects? What would be the consequences of your company losing them as a client and finding out that if was your fault? 

**All things considered, I'd suggest you take a pass unless the money on offer is sufficient to pay your wage if and when you get fired.**