Halflings, despite their dex bonus, would cap at 18 str, 19 dex, 20 con, and 18 wis. If you want to be really ruthless, you could drop str and wis to 17 I guess. Either way, it's unlikely to affect many builds.
TBH, if I were going to include STR racial caps for halflings it would be much lower than 17. These creatures are 3 feet tall, thats under a metre. The difference between the brute STR of Humans and Halflings or even Half-Orcs and Halflings should be massive.
Below is a rough draft of what I'm thinking.
I would rewrite the abilities and proficiency (skill) bonus increasing the table to +7 or +8 and make the ability bonus flatter ranging from +1 to +3
So
12-15 +1 ability bonus
16-18 +2 ability bonus
19-20 +3 ability bonus
Cap Halfling STR at around 12, Humans at 18, and Half-Orcs at 20.
Give Half-Orcs an additional Quality
Brute Strength - Carrying Capacity of Half-Orcs is STR score multiplied by 20 (as opposed to 15), while their Push,Drag & Lift capacity increases 40 x STR score (as opposed to 30). Furthermore damage from STR score is doubled. i.e. 19 STR yields +6 damage.
Give Halflings additional Qualities
Athleticism - Halflings are agile, surefooted and athletic as such they gain a +1 bonus in climbing and jumping for every 3 points of STR.
Halfling Nimbleness - In addition to the trait on page 28 of the PHB, halflings gain an inherent +1 bonus to their Armour Class when in combat with creatures of Medium or greater size.
Natural Talent - Rock pitching is a universal sport of Halfling children and such Halflings gain a +1 racial bonus on attack rolls with thrown weapons and slings. Furthermore the potential for a critical is increased by 1 from such attacks. i.e. A dagger thrown or a stone slung by a halfling may cause a critical on a natural 19-20.
Ofcourse the racial ability caps may be increased or even undone through narrative means - such as my example upthread (post 403).