I feed boiled barley morning and night and always ad about a cup of molasses to the pot mix it and then divvy up the barley to each horse as I see they need it. So mine get it twice a day. Everyday.
Firstly I dont think it hots horses up. We have been feeding it to all our hacks and ponies and racehorses over the years and I can never recall one going silly because of it. However in saying that if a horse is getting fed well and worked well then naturally they will become more fit and thus have more energy and perhaps be a bit jumpy or active or whatever.
I have also been told that feeding molasses in small doses on a regular basis is great for keeping sand and other nasties out of the gut system therefore reducing the risk of colic and ulcers. Numerous old school racehorse trainers have told me that so thats good enough for me.