First of all, congratulations on your progress. If you're losing weight and increasing mileage and gradually improving your 5K times, you're doing things right, so don't change too much. More information would help us give you advice. In particular, how old are you, what are your pre-running exercise and injury histories, and is your 23:13 5K a solo time trial? The older you are, the more you need to be concerned about not overloading yourself and avoiding injury. I assume being 6'10"" and 94kg means that you need to be more careful than smaller people too. Whiskers' post seems more appropriate for 60kg 16-year-olds, as they improve quicker, recover faster, and are less susceptible to injury than the rest of the population. One workout and one long run a week is safer for the rest of us.
As for your question, nobody can be sure how quickly you'll improve, except that if you get injured you won't improve. Sometimes people say that most runners improve for their first seven years running; that varies greatly from person to person, but it does suggest that you can keep improving for a long time. If you're going to keep doing tempos every week, the only other fast running I'd do for now is adding 4-6 strides - moderately fast but controlled runs of 50-100 meters with full recovery - one day a week after an easy run. You may want to mix up your faster running - one week run your 40-minute tempo, maybe a bit slower than 8-minute pace though, another week try a 20-minute tempo at 8-minute pace, and another week try 4 800- or 1200-meter repeats at a slightly faster pace with 3 minutes rest (maybe walking at first and then jogging after you've done the workout a few times if it feels ok). Don't run the repeats all-out, though. The McMillan calculator can give you an idea of paces, but if they feel too hard, don't push. 40 minutes at 8-minute pace does seem to be long for someone at your current fitness level, though if your 5K times are solo time trials and not races, your actual fitness may be better than the 5K times suggest (i.e., you would run faster in a race). Also, when you're increasing mileage, add it to your easy runs, as your long run seems long enough already even for 40 mpw. Good luck!