Yes, if no other changes are made
That's... an interesting question.
The first thing to do is to get rid of the half-level scaling. That gives you a huge framework to add buffs around and even a desired level for those buffs.
Secondly, Replacing +1 to hit and damage with +2 to stat will have two side effects:
Ripping out the half-level component of the game and replacing it with stat mods on top of additional items will lower skill increase rates (due to only stat-based skills being buffed), will maintain to-hit where it should be, and will increase average damage.
In order to balance the game, therefore, you need to remove:
- Half-level bonus to attacks, skills, and defenses
- Enchantment bonus to attacks which scales every 5 levels
- Tier and level 4 and 8 stat increases.
So, to test:
Normal stat progression:
l stat mod mod+half
1 18 +4 +4 +4
6 19 +4 +7 +9
12 21 +5 +11 +14
16 22 +6 +14 +18
22 24 +7 +18 +23
30 26 +8 +23 +29
By simply replacing half-level adjustments with stat-enchantment adjustments, you get a comfortable +15 bonus (which translates to +30 for stat) to play with by the end of epic. By replacing that and normal enchantment bonuses and level & tier upgrade stat bonuses, you get an even more absurd +21 bonus by the end of epic, which translates into upto +42 stat.
This, of course, leaves secondary effects and damage in question. Consider the Ardent's Demoralizing Strike, Agument 2: Hit: 1[W] + Charisma modifier damage, and the target takes a penalty to all defenses equal to 1 + your Constitution modifier until the end of your next turn.
By epic levels, that debuff instead of being a -8 (at 22) will be a -23. I wish I could say it make a different in the balance of the power, but given typical accuracy, the debuff doesn't need to stretch more than around a -8 or so. (-8 to 12, depending on character builds...)
You'll also have problems with powers like "Drunken Monkey":
Hit: 1d8 + Dexterity modifier damage, and you slide the target 1 square. The target then makes a melee basic attack as a free action against one enemy of your choice. The target gains a bonus to the attack roll equal to your Wisdom modifier.
Or "go ahead, autohit." Still, +8 guarentees about the same thing.
The real problem is when you apply the modifier to damage. Consider the Artificer's magic weapon:
Hit: 1[W] + Intelligence modifier damage, and each ally adjacent to you gains a +1 power bonus to attack rolls and a power bonus to damage rolls equal to your Constitution modifier or your Wisdom modifier until the end of your next turn.
+23 to damage as a bonus... is gamebreakingly silly.
There is a solution however.
You must also scale up monster hitpoints
Average Monster HP is (8*level+24) And an average striker should take down the monster in 4 rounds. Therefore, we can derive at each of the key levels, the damage delta and make retroactive changes to monster HP.
Happily, there is already a normalized equation for expected striker output here.
We must consider:
Non striker minimum output of Level+3
Striker average output of 2*Level+6
Broken output of 4*level+12
This mod functionally applies half-level to damage... Computing that on top of striker *2
lvl 1* 2* 4* mod
1 4 8 16 8
6 9 18 36 21
12 15 30 60 36
16 19 38 76 46
22 25 50 100 66
30 33 66 132 81
While that doesn't increase the brokeness (beware of multi-attackers, BEWARE!), it does suggest that a monster requires 12 more HP at level 6, and 24 at 12.
Remove all normal sources of bonus in the game that would otherwise be covered by intrinsic enchantment modifiers (the +1..+6 of items) except for heavy armor, the half-level modifiers to skills, defenses, and attacks, and the tier and level based bonuses to stat. Replace these with your house-ruled item based bonuses. You will need to increase MM3 monster HP by roughly Level*2, but testing is required to see if that is the case. You will need to deal with extra sources of scaling damage beyond that in some kind of systematic way. I recommend testing combats at each level at this starting modifier and measuring how long it takes for characters to drop a monster, then adjust HP accordingly so it lasts the expected number of rounds.