I've heard that significantly exceeding your target heart rate during aerobic activities can have a detrimental effect (possibly stripping you of muscle and or retaining more fat)

I am currently weight training four times per week (one day on one day off ie, mon,wed,fri,sun) in the afternoon/eve and doing 30 minutes of cardio at my target heart rate for my age and weight five to six mornings a week in addition to following a six meal a day "glycemic index" type diet in an effort to gain/retain muscle while losing fat and generally increase my cardiovascular health.

Target rates are established for the ability to continue for long periods of time, therefore increasing the value of the excercise. Exceeding this rate simply means you will not be able to maintain that excercise for extended periods. There are different theories out there that will tell you many of those typpes of things as you mention but most do not hold water. It has long been said that short burst excercise has no value but many athletes that play tennis or raquetball show that to be untrue. The best overall diet plan I have had luck with is simply go ballanced on all meals. Roughly equal carbs, proteins, and fats. Combined with excercise this will get you as far as fast as any diets for sale.