One huge benefit of keeping boxed objects on odd values on the low bits is that you can implement addition/subtraction of the integers without removing the flag

One huge benefit of keeping boxed objects on odd values on the low bits is that you can implement addition/subtraction of the integers without removing the flag bit, doing the operation and then re-adding it, instead you can just add 2 values with the regular add instruction and then use it (since the lowest bit won't change). On the other hand having the pointer offset doesn't do much of a difference since heap values will often be accessed with an offset (and offset-load/store instructions are more or less for free in most cases so subtracting a one to an offset doesn't change the cost)