If I were to leave it strung for a few days, or longer, would it lose an appreciable amount of weight? Would it become less efficient?
Leaving a selfbow stung for long periods of time will ruin it. The bow will take excessive set as the wood cells will be crushed and damaged from the prolonged strain. Fiberglass doesn't have memory, but wood does.
Reducing the weight is easy. What weight do you want it at?
To reduce weight, you need to remove wood. If the tiller is fine, then take a cabinet scraper and give each limb 20 full passes on the belly side of each limb, then check the weight and tiller.
If the tiller gets thrown off, then you have to correct for that (in that case, things get tricky). If the tiller stays "in tune" you can continue to take 20 full scrapes off each limb with the scraper until it feel right for you.