I prefer IM for LR3 or DES. It does hit the blood fast so works systematically but I have noticed site enhancement. The way I look at it try it as it's not going to go against you in any way. If you have weak bi-ceps inject them everytime you dose lr3. It will still be absorbed systematically no matter what. If your bi-ceps improve due to the site injecting then it's a bonus.
It's worth mentioning site injections can improve the muscle in a variety of ways. One way is the improved mind muscle connection you will get with that muscle especially if you inject pre workout. Some people find it harder to target certains muscles such as the lats or chest. Inject lr3 (or anything) in your lats pre workout and you will feel them constantly so they are easier to engage. I would recommend hitting them for a few sets even when not training them that day. Obviously on your back day hit them hard.
So if you have a weak body part you could improve it by adding in site injections of peps and frequent low volume training.