Judging from the literature provided below, it seems that serum concentration reaches the same levels as IM, and peak levels are attained by day 2. I'm struggling to find serum concentration graphs for IM right now, however, but from what I'm reading is that they're roughly the same. That's sort of the point to the following research. This was based on a study of weekly subq injections with 5/8" needle at 50mg and 100mg.
Correct me if I'm wrong here, but I believe the larger benefit is that you can do more frequent injections with less pain, scarring, bruising, etc. The frequency is the key point here, I think. That's what keeps your serum concentration stable.
If you're comparing how you felt on IM, how often did you do those, and at what dosage?