I think there's no point in mixing the two as a single form of offense in enough + these require completely different approaches.
As a sniper, you're supposed to shoot from afar and maintain distance, using the enviroment and CC. Someone with a shotgun means short range combat. Also valid, but totally diferent and excludes the other. This way, you're not going to use Shooting Spree ever again after you start combat with it, as in order to speed up the cooldowns, you need to keep using your rifle. Same for Aim Shot.
Interloper is missing. Hit and Run is also more suitable due to granting a lot more MP in fights and providing it consistently in a passive manner without being a one-time ability with cooldown. Would save you a lot of stat points and feats as well keeping AGI at 7 (for Interloper), and ditching Sprint, Expert Sprint and Blitz.
So if you're fine only using your rifle to delete two enemies in combat for free (which is not bad), then you don't need to even specialize on Shooting Spree more than 1 point on shots, activate it, take your Snipe and Aim Shot and forget it for the rest of the combat. But a sniper uses his rifle for all the kills to circulate these abilities as fast as possible, backed up with Tranquility-fused Increments. Using Ambush from the dark to ensure crits and boost accuracy (which is easy to set up when weapon range is not limiting you) and keeps distance between his enemies by kiting with Hit and Run, taking cover behind corners and should someone get close, Flashbang and disengage. Stealth, then repeat.
While you're using your shotgun, your sniper abilities are parked - often including Ambush. Yet Kneecap Shot is a good way to halt incoming melee's, but still I see no point in specializing two ways on the cost of not excelling in neither while you can only use one opposed to speccing in a single form of offense better and have more supplemental feats to elevate it's versatility further. You cannot reach 4m height if you have two separate 2m long ladder. The key is to set up something that can be effective and be kept up long-term, against continous onslaught of enemies. Circulating Shooting Spree every round, using Aim Shot and Ambush, finishing rounds in stealth to Snipe the next one behind corners and just generally making every shots count and contribute to lower cooldowns is something that I see as the way to go with the rifle to work best. The more you shoot, the more you can shoot. That's why I think snipers should stick to their rifle exclusively and collect abilities that allow them to use it fluently and to greater effect - especially when the AP cost is very similar due to shotguns not being able to use the Rapid Reloader.
At least that is my opinion, I remember a few people recommending these two weapons together, but I don't think they knew the potential of just how effective and fun a dedicated sniper can be.
