Enthusiast Looking to Upgrade Either Lens or Body

Hi all,

First time posting here and I have been torn for weeks so I decided to try posting here. I have read dozens of reviews, articles and other forum posts and I am at a quandary.
My current gear-
Canon XSi which I have had and loved for 4+ years but am starting to feel its limitations in terms of ISO, processing power for continuous shooting RAW and FPS
Tokina 11-16 f2.8, Canon 18-55 kit lens, Canon 55-250 kit lens
I primarily use my camera in the outdoors on hiking and mountain climbing trips. It is often subjected to bad weather and getting wet, but this has not yet been an issue with my XSi. I am not really willing to sacrifice image quality to get a true weatherproof camera. I mostly shoot landscapes and action shots of climbers, so I spend most of my time at the extremes of my focal ranges. Low light is also often a critical issue for me when shooting on the move in at dawn and dusk. Because I am a climber who is also a photographer and not the other way around, I must keep up with my companions and so a tripod is not usually an option for me unless I am shooting stars which is also a favorite of mine.
I should also mention at this point that I mainly take photos for the fun and to share with my friends. I do not usually blow them up huge for my wall, though I would like to expand more to doing something with my photos.

I have long found that I am at the limits of what I can accomplish with the kit lenses and so I had been debating what to replace them with. I have been most eyeing the Sigma 17-70 f2.8 as a direct replacement for my kit lens or a sigma 18-250 to replace both kit lenses. I know I would sacrafice a ton with the superzoom and I am hesitant to do so especially since I spend so much time at the ends of my zoom range, but the ability to not have to carry and change two lenses in the alpine is very appealing. I cannot decide if the distortion and softness at the edges out ways that. The Sigma 17-70 has less of the negatives with image quality and the low light performance will be a huge boost as well, but it is a direct replacement in terms of range for my 18-55 and so I still have to carry 2 or 3 lenses if I want the full range.

My quandary reached a whole new level when I stop to consider if maybe I should be thinking about replacing my camera body. My XSi is nearly 5 years old and it was a little dated when I bought it (The XTi had just come out). It also has some significant sensor dust that I have been unable to clean myself and didnt feel it was worth having professionally cleaned when I might end up replacing it. I bought it in college when funds were minimal and I had never used an SLR before but both are less of an issue now. The Iso is limited to 1600 and I can only take about 5 continuous photos at raw before the buffer fills. After a quick scan and some reading I had been eying the T5i or 70D as potential replacements. As I said, I spend a lot of time shooting in low light and stars, so the higher ISO might be a huge boost for me. The ability to shoot video would be neat, but not sure how much time I might spend with that.

My budget for an upgrade of any sort is around $1000. It isnt that I dont have the money, but more that I put my gear through a lot in the mountains and I would rather spend less so I dont have to feel like I need to baby my gear when I go out hiking, fully accepting that it might break and it would be affordable to replace it.

I am well aware that prime lenses would be a much better jump in image quality and low light performance, but I find it hard to sacrifice the ability to zoom given that I often have one chance at a good photo as my subjects are always on the move and so moving myself to get into position is rarely an option. Also I frequently find myself on a cliff or rock wall and moving isnt an option at all.

Basically I am looking for anyone with any of these lenses or who has done a similar upgrade and to get your feedback.