The suspension starts to take a nose dive around 75k so I'd say at 100k it's definitely in need of a refresh. The sport suspension is better than the standard but in my opinion it was never night and day different for me. Noticeable but it doesn't make it an M car. Driving an S5 right after doesn't do the e91 any favors since these are two different animals. A 335i m sport or e46 m3 are better comparisons. The V8 in the S5 is nice and sounds great once it's opened up. They don't have the same issues as the earlier 4.2 in the b6/b7. By the B8 chassis the S cars basically became a spicier version of a normal car while the RS cars were direct M competition so consumables aren't that bad as far as price goes. With that said the S car is probably still going to cost more than an e91. Also, you're still dealing with a German V8 so you still need to have some healthy fear of financial disaster at any moment.
The e91 is a great daily utilitarian vehicle but the n52 isn't going to add any thrill to your daily commute while the V8 in the S car will definitely add some excitement. You could make the e91 a great handling great looking car but it'd never compare to the S5 overall.