The 1C/1D would work really well with the E400XLB. I get it’s a short journey to use such big buses but the B9TLs and E300s are absolutely packed for the majority of the peaks and even beforehand. They would definitely earn their keep as it’s a busy busy route. Then they could be used on busy Caley routes throughout the weekend.
I do like the E300s they’ve grown on me but my general opinion on them is that everything about them is so light and flimsy. The E400MMCs aren’t half as flimsy as the E300s feel. I always think the smaller wheels and lack of double glazing make them rattle more and bounce more on a journey. Plus on routes like the 1, 2 and 3 which are really busy I think they’d benefit from buses like the B8RLEs or perhaps Mercs like McGills.