Imperial stout high FG

• Problem/Question: OG 1.098, FG (after 4 weeks) 1.040, FG seems high.

• Malt (BIAB): 6 lb Maris Otter, 4 lb Thomas-Fauwcett Pearl, 2 lb Special B, 2 lb 2-row, 1 lb chocolate, 1 lb flaked oats, .5 lb each of roasted barley, Carafa III, and caramel 120.

• Hops: 1.5 hr boil to reduce volume, 1 oz Northern Brewer @ 45 mins, 1 oz Zythos @ 15 mins, 1 oz Centennial @ 2 mins.

• Yeast: 2 packets Safale US-05, re-hydrated. Pitched into 3-3.5 gallons of wort.

I know airlock activity is not a great indicator of yeast activity, but there was a great krausen after a few days that was essentially gone after a week. After exactly 4 weeks, took an FG reading at 1.040. This puts it at 7.5%-8%. I'm very happy with the flavor, and I can live with the ABV (was hoping for high 9s or 10), but I'm wondering why my attenuation is so low. Should I go get some wine yeast and re-pitch? I don't taste a lot of residual sugar, so maybe my efficiency was just terrible, though I tried compensating for that by boiling it down. Ferm temps were not very well controlled either; wondering if Safale can handle swings well (was between 62F and 75F at all times, usually in the 69F-73F range).

Any critiques, told ya so's, calls of dumbassery, and, of course, advice, is very welcome.

Thanks.