Congrats on a fine acquisition. I have had one for a month and finally shot it this past Friday. I came across it in a shop and really didn't know what it was. When I saw that it took AR mags I was sold. There had been a thread here about pump rifles so I was immediately intrigued. $349 seemed more than a fair price for it and since I had some money burning a hole in my pocket I came home with it. I believe that it was a police trade-in. It has a few minor scuffs on it that seem to be consistent with having been carried in a cruiser. Mine has the Wilson Combat rear ghost ring sights, front XS sight, but no rail.
The first thing that I did was to replace the butt stock with a Butler Creek steel folding stock. This is one rock-solid unit. The problem was that the 7615P uses the 8700 lightweight receiver (20 guage I believe) and the folding stock was for a full size 8700. I suspected it when I bought it and confirmed it when the fit was way off. It took major alteration with a Dremel and drill press work to the internal aluminum housing, but the stock is on rock solid and I am 100% pleased with the results. Altering a $90 stock as I had to is not for the faint-hearted. That said, the Butler Creek stock is exceptional and well worth the time/risk. It does add weight to the rifle but I wanted something that could make the rifle a bit more protable in terms of length, so the trade-off in weight is well worth it.
The rifle is a blast to shoot. Key to proper function with feeding and ejection is a very positive and strong cycling of the pump. If you baby it or ride it slow your round may hang up. After realizing what was necessary for cycling it wasn't an issue. As to accuracy, I only battle sighted it outdoors at 32 yards in a pretty stiff wind. Overall the Wilson Combat ghost ring sights are going to limit this rifle for any precision work. I'm okay with that as that isn't what this rifle is about for me. Once I got it sighted in five shot two to three inch groups were the norm. Note that with the Wilson Combat sights the set screw for the ghost ring must be loosened enough for the sight to raise up enough up off of the sight mount so that it disengages the grooves otherwise you will be limited to adjustment for windage only.
As to the ghost ring sights, the apperature is large as one would expect. The front XS sight post is also on the wide side. Any level of significant precision out past fifty yards is going to be limited. While I only shot out to 32 yards I would expect my results to hold out to fifty, but out at 100 yards that front post is pretty much covering an eight inch target. I should also mention that I was shooting Federal bulk .223 ammo that I believe was 55 grains. I was also shooting an HK SL8 which is usually a tackdriver but my results were pretty dismal for that rifle with the Federal. When I switched to some Lake City Green tip (which is either 62 or 66 grain, can't recall which at the moment) everything tightened up with the SL8 very nicely. I didn't get back to try the Green Tip in the 7615P but I would have to predict that things would have gotten even better.
I suspect that with scoping the rifle longer range "precision" shooting becomes more viable. The ghost righ sight set-up is going to limit accuracy to a large degree, but for quick sight acquisition it's hard to beat. Even with the ghost ring sight I would feel confident in being able to hit an six inch gong consistently at 100 yards and maybe out beyond that although I can't say for sure as I didn't attempt it. One way or the other this is probably a 100 to 200 yard rifle depending on how it's sighted and it's intended purpose.
The best part of this rifle is that the grin factor is sky-high.
Which means I have plenty of ammo, and mags for this rifle, without spending any more money. 
