BTW is 173 wpf + NOT heirloomed Strong Bow enough to get smallest available reticule? Or i need 180 wpf (it is difference between 2ps 8athletics 8wm 15/24 and 15/27 3athletics 9wm)?
I don't know the answer to your direct question (if 173 gives you the smallest reticule) but I can offer some advice, which is DON'T GO FOR IT!
If you only have 5 power draw any benefit you might get from being able to hit someone from further away with the increased wpf is completely negated by the fact you'll do such a small amount of damage.  My last gen I used a fine Kherit bow and went the following build:
Level 30 (4 420 577 xp)
    * Strength: 18
    * Agility: 21
    * Hit points: 53
    * Converted: 8
    * Ironflesh: 0
    * Power Strike: 3
    * Shield: 0
    * Athletics: 7
    * Riding: 0
    * Horse Archery: 0
    * Power Draw: 6
    * Power Throw: 0
    * Weapon Master: 7
    * One Handed: 1
    * Two Handed: 1
    * Polearm: 1
    * Archery: 164
    * Crossbow: 1
    * Throwing: 1
I was a big fan; the 164 wpf was plenty for medium long/long range shots, and once people went medium-medium close I could start aiming at heads.  Having the spare points for athletics was a godsend and I think you'll be easy food if you only go 3 ath.  In the future I think I could cut down weapon master even more(down 1 or 2 points) to hit harder with my arrows.  Once people start getting to that medium range even the 148 wpf is plenty, and you'll pack a much harder punch with the shots.  
My two cents; do whatever you want but I think any more than 7 wm is probably a waste of your time as an archer unless you're going to go 3 full quivers of arrows and just spam all day from a hill for like 5-8 damage a shot.