Ok...very general impressions.
I am not familiar with Grim 'n Gritty rules or the Conan PRG. I am familiar with Midnight.
That said, Iron Lore made a terrific replacement for the Midnight character generation and magic system and we use Iron Lore in a Midnight campaign for the playtest. And my group preferred Iron Lore to Midnight for creating characters for our Midnight campaign. (Yes, I can hear the cries of 'Heretic!' even now.
I can't compare with Conan RPG, not having seen it, but we modified the Conan Unleashed (old 2E) adventure and used that to do a Conan mini-campaign. And Iron Lore did a far superior job than core D&D for simulating Robert E Howard's Hyborian age world.
how would rate the overall high level play-balance in IL vs D&D / AU[AE]? Fun factor? Do rounds of combat take much longer to aducate in IL with all the stunts, damage reduction and combat options? Is it fun or can it get a little tedious?
IL characters are powerhouses in their own right at high level. While you will have to tweak a bit, IL characters can play along side D&D/AU/AE characters in a high level campaign. The tweak is not so much characters as it is addressing fundamental core assumptions - like armor class as DR and whether IL characters can have free use of magic items. (The default assumption of IL is that magic, while powerful, is rare and magic items are rare as well).
While IL adds much to the game as the stunts and combat opinions (there is far more than that ....but there is that NDA thing...), it is modular and you can use what you like. There will be a initial learning curve as IL characters have access to more options than a standard core D&D character but it is not overwhelming and after a play session or two, you don't notice any meaningful slow down in game play. And certain things, like AoO in particular have been simplified, which helps keep the game clipping along.
Magic can be powerful but it is rare. And risky to use. The consequence of a failed spell or a spell interrupted provides the check and balance to high level play which tends to be dominated by magic in core D&D and AU/AE.
And about the fun factor. Core D&D and AU/AE tends to have the players looking for magic or accounting for magic in their character developments, since magic is a significant part of the game. Hence, the contention by many that high level characters are more of a sum of their magic items than their character abilities. Not so in IL. When you have stunts and challenges and you are not sweating the magic items (because they are rare and dangerous), the game can be more fun.
Case in point. Most everyone had done the tavern brawl thing before. In core D&D, this tends to be roll to hit and roll unarmed combat damage with some descriptive text by the player and/or DM.
The environment is a big factor in IL. An IL tavern brawl has mugs or food being thrown, tables being flipped, chairs being kicked into people or being flung, things being used to swing from, bodies being hurled, ropes being used to clothesline people, people slipping on spilled drinks....basically, the type of stuff you would see in a Muskateers movie. IL has the rules structure to easily allow this sort of thing in normal game play.
Fun. And if you wanted a tavern fight, which one would you want - roll to hit and do damage with some minimal explaination of how the cinematics worked out. Or would you rather do a leaping roll on the top of a table, kick the chair leg out from under the loud mouth lout sitting there, while flipping the table into his nearby friends sitting with him.... and know that the means to adjudicate or create this stunt is supported in the rules.
Or how about getting a positional advantage by 'climbing' onto that raging troll or bullette and conducting combat from the relative safety of its back... IL allows this.