Sinject is not the ideal injector. When I first started hacking a few months ago, I used sinject and I got detected half the time. Then I switched over to perx and had never gotten detected even once. After possibly hundreds of injections, I still haven't gotten the hackshield detection message again.
While perx won't inject properly sometimes but I can say that it's 100% safe from hackshield. If it doen't inject properly then just exit and inject again.