The biggest problem you're having is bound to be the cabinet you're running it through. Your assertion that a 5150 will always sound like a 5150 regardless of which cab you're running it through is only true to a limited extent. The signal leaving the 5150, if sent to two cabs, should be identical. The sound that you and the audience hear from each of those cabs (assuming they're not identical cabs obviously) can be quite different. If you're thinking that you've heard lots of people play 5150s through different cabs and sound pretty similar, maybe those cabs weren't as different as you think. There was always discussions amongst the Mesa players when I had a Mesa halfstack about which cab was best for the amp. The differences in their recorded sounds really weren't all that different though regardless of which cab they were using. That was because the cabs they were arguing about were actually all pretty similar. For the guys that liked V30s, they were comparing Mesa V30 4x12s with Marshall V30 4x12s and other brands of V30 4x12s. Of course the dimensions, materials, and construction methods of the various cabs imparted some slight differences in the tone and response of the cabs, but the differences were fairly minor differences in essentially the same tonal flavor.
The speakers play a HUGE role in the tone that you actually hear from an amp. They play a far greater role than most players realize. If you were in an FRFR situation, the difference is easy to see. Take a well known combination like the Plexi and Greenbacks and dial in a good Plexi Marshall tone. Then without touching the amp controls, switch to the 4x12 G12T75 cab or anything else not typically used for that classic Marshall sound and you'll discover that it isn't a slight change in the sound but a radically different tone that isn't anything like the classic sound you were looking for and had with the Greenback cab.
You're looking for the 5150 metal tone, so unless you want to go FRFR your easiest course of action is going to be getting the kind of cab that 5150 metal players typically use. If you go FRFR, you'll just need to find the IR that has the sound you want.
If you want to stick with the gear you have, that's great too, but you need to understand the limitations of the gear that you're using first. That doesn't mean you can't achieve the results you want, but you may have to try something pretty radical to get the tone you ultimately want from the rig. It is highly unlikely that it will be as easy as dropping in the 5150 sim and turning a few knobs. I would be expecting to work with at least a peaking filter and some additional EQ to get exactly what you want.
I don't have your cab, so I don't know exactly what sound you're getting out of it versus what you want. Try this though... Place a filter block before the amp and play with this: a 6dB boost peaking filter @ 725Hz, Q 0.35. Those are settings that Don Peterson uses in front of the Caroll Ann model to get a Two Rock sound with his amp model settings. I tried it for fun one day and it added an interesting "snarl" to the tone as I remember. It definately changed the character of the sound quite a bit in an interesting way. Of course, those settings probably won't be exactly what you need for your tone, but it might be a starting point that makes the lightbulb come on for you as to how to find what you're looking for. The cab you're playing through obviously doesn't have the "snarl" that you want naturally, but that doesn't mean you can't try to add it in there.
Lastly, for those saying you have to upgrade to the II, I just don't buy it. I have yet to find a sound with a similar amp model in the Axe-FX that I couldn't get, period. Others have had the same experience. Of those who have been able to get the sounds they wanted and later upgraded to the II, they have reported that they can get them a little easier now. That's a great benefit no doubt, but the fact is that they could get the tones with the Gen I units as well. I can get the sounds I want from the I, but I will probably upgrade at some point for the added processing power allowing me to do some even wilder things. Since 11.0, I don't have to jump through a lot of hoops with my tones to get what I'm looking for anyway. Before that, there were a number of tricks I found helpful to get the amp sounds I wanted. Since 11.0, I've found it very similar to dialing in the real amps. I've owned a number of the amps in the unit, so I'm not just saying that.
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