I've only tried the low cholestrol meal and it was substantially similar to the regular meal (no better), just with low cholestrol and fat substitutions. Rather interestingly the meal was labeled with 3 different meal types (smart catering, using the same meal to satisfy 3 different special requests -- but it means that the special meals are more restrictive/healthy than you might expect). Don't order it for the flavour. If you are pre-diabetic, then the diabetic meal is precisely what you should order.

BA won't op-up anyone with a special meal ordered. I think QF is the same for op-ups; paid-for upgrades are a different story and not affected by having a special meal. Most airlines will not select pax with special meal for op-ups as this creates 2 problems (no special meal in the higher class, and no regular meal for the lower class -- most airlines don't board extra meals in the lower classes, only in F and J).