I've owned a 5150, 6505+, 5150 II, and FJA 6505. Every time I got one thinking the same thing, "Alright, THIS time it will sound like I expect it to--the way people talk about it." And, everytime, I sold them, disappointed. They have insane amounts of gain but have a certain fizz and mud about them that is just unbearable to me. This problem is well documented by many 5150 users and some of the best recordings of them don't have this. That's why I kept buying them; people I heard using them had badass tone, but it just never worked for me. I don't know if that's due to improper EQ (I tried about everything people suggested) or what, but it wasn't cuttin it.
That being said, I dialed in the Ultra XL (stock) perfectly in about 3 minutes, literally. It has the tight, crisp, staccato nature I'd always wanted in the 5150s. Similar amounts of gain, similar tone, just way tighter and more defined. I definitely prefer the Ultra XL to it. So yes, they're similar, but I think the Ultra XL is an upgrade for sure.