"I am the typical low hour PPL who has altitude deviations of +/- 100 feet and heading deviations of +/- 5 degrees."
No one's perfect my friend. From what I've experienced, you are allowed a reasonable buffet when it comes to such deviations during instrument training. The whole point of this rating is to ensure that you can control the aircraft with that level of preciseness above that of a pilot without an instrument rating, not to mention the added level of safety which come from having an IR. We all know through statistics that the number of weather related accidents go down with IR pilots as oppposed to non IR pilots(Fair_Weather_Flyer and FlyingForFun you can correct me on this one).
But back to the point. What happens when you have to fly with 18knot gusting winds for example? Certainly you don't expect to keep that needle dead center on that altimeter do you? {without damaging something of course
}. But on a more serious note, true it's extrememly challenging that's for sure, but others have done it and so can you. That's one of my personal mottos which have gotten me through training thus far...