From my experience with my engi I prefer the class specific weapon. While in pl as you said dps is overrated in sl it seems to be the opposite. The difference in weapon speed is significant. Plus from my informal testing the "boxing gloves" seem like they have a stun like proc and can occasionally cause the enemy to pause for an attack or two. I just haven't seen any thing at this point to make me think rifles are a good choice for an engi. However I'm really reckless with mine. I always open with transference and end up doing a lot of tanking as a lot of commandos seem a tad too concerned about stats and unwilling to take point. While my play style has led to my share off walks of shame back to party it's fast paced and exciting with all the HOT and DOT ticks going on there is never a dull moment.