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.