Sounds like a awesome build. I really want an Impact now for some reason, guess the buzz is getting to me i need to get my hands on one.
I've not used the small mark 5. The 5-25 is wonderful for the money and you'll not fine many complaints on it at all. Truthfully for the price they can be had at, it hardly warrants any. I've wanted the smaller mark 5 for some time but have put money elsewhere so i just haven't picked one up. I'd say if you can get it around 1500 or so with the reticle of choice, that it would be incredibly hard to pass up. I've seen some complaints of lack of resolution and some CA. Both of which where great attributes in the 5-25, it had very little CA, great resolution, and edge to edge clarity. The contrast was not quite up to snuff compared to pricier competition but that doesn't detract from it IMHO. Though it seems that most of the Mark 6's short comings have been ironed out and given how small it is the sacrifices made for the size and weight are probably worth it. I know ILya seems to be very impressed with his. Redneckbmxer is stoked with his so much he's already gotten two more or something like that. I went Kahles across the board despite it costing me extra. Mainly because after using the SKMR3 i fell in love with how clean it is for hold overs and i honestly wasn't sure i'd ever feel a better elevation turret.
Though if weight is your main concern you'll be best combining a light scope like that with a smaller barrel profile. Mine is still nearing 13lbs or more depending on the config. Didn't nearly reach the 9lb mark i wanted, though given it's purpose i'm okay with it. Dad's 6.5 SAUM is right at 9lbs without the bipod and suppressor. That was with a 3B fluted and a MCS T bedded.