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** and can you afford to sue them if 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.**