Wyeast high gravity attenuation

I brewed my first really big partial mash recently - an Imperial IPA that I plan to age on bourbon soaked oak. I'm going for the Dogfish 90 minute/Burton Baton taste.

My OG started at 1.100 for about a 4 gallon batch (I didn't want to dilute any further since I was really hoping to hit that magic 10% number, lol). I pitched a Wyeast 1098 British Ale 1L starter and pumped pure O2 through. Fermentation went fine and quickly (as 1098 always does - I use it all the time) - down to 1.035 (65% attenuation so far) but has stopped there for the last 3 days. 1098 is supposed to have 73-75% attenuation - so I was hoping to get it into the 1.025-1.028 range.

I was planning on adding a little Safale T-58 a few days before bottling since I am going to age it on oak and the alc content will be close to the upper limit of the Wyeast 1098 tolerance. Would there be any benefit to adding some T-58 now?

65% attenuation seems low. If it were me I would add half the T58 before bottling to ensure it's not going to eat more sugars than you expect and you end up with gushers, or worse; bombs.

I'd probably add half the T58 packet to a couple of liters of average wort (about 1.060) and let it start fermenting (a couple of days), then add a couple of liters of the IIPA to it and let it ferment for a day or two, and then add all of it to the main batch and see what happens.

If it ferments lower than 1.035, then you saved yourself some real problems. If it stays where it is, you know it is ready to bottle.

I did this to a 1.100 barleywine, and it took it from 1.025 to 1.017. Certainly saved me from a big mess with exploding bottles.