Don't worry, the 13kg limit is usually more than sufficient - although they put on weight rapidly after birth, this slows down as they get bigger.
10lb 4oz at birth would put her roughly on the 98th centile, but children don't often stay on their birth centile. Do you know how heavy or long she is now, or what clothing size is she currently wearing? Even if she is on the 98th centile, that would predict 13kg at around 17 months old, so you have plenty of time unless her head is getting to the top of the seat.
The issue usually with Maxi Cosi is the height limit, though I'm not sure which exact one you have, G-Cell is the name of their side impact protection system which many of their seats have. Do you also have a base for your seat? If you know which base you have, sometimes it turns out cheaper to get the toddler seat which adds on to that base. Have a look at this chart. If the compatible toddler seat is more than £200 though, a standalone seat is better value.
For buying a whole new seat:
There are basically two major decisions to make for the next stage, and from there that narrows down your options so there is less to choose from.
First of all, the seats for the next stage go up to either 105cm with a weight limit of around 18-20kg (approx age 4, but might be age 3 or so if she is on the bigger centiles). As she is bigger for her age, you will probably need another seat with harness after this, before you move to high back booster. That is a bit of a pain if you only plan on having one child. If you think you'll have another child, it's not bad because you can pass the seats down in turn and maybe save doubling up on them being in the same stage at the same time.
Or you can get seats up to 125cm, with a weight limit of 25kg+ depending on the seat. This is approx age 6, meaning you can go directly from this seat to high back booster.
To simplify things, ignore the seats which go up to 150cm - they are usually not the best option anyway.
Secondly, at 8 months she still needs to be rear facing. So the next choice to make is whether you want a seat which will stay rear facing for the whole of the next stage (up to age 4-6) - this is the safest option, and these seats tend to be the longer-lasting ones as well as compact in the car with good leg room for the child.
Or, whether you prefer the idea of a seat which can be turned forward facing later down the line. Many of these seats have a spin function, which people often like, but most of them are only suitable up to the 105cm mark. If you are keen on the safety benefits of rear facing, a two-way/spin seat can also cause a problem in that it's almost a bit too easy to turn them forward, especially if your partner/a grandparent etc is keen to put them FF earlier than you'd like.
Eventually after she moves into the final stage at about 4-6 years when you feel she is ready, she will be FF anyway but that's another type of seat and I'd wait until you get there.
Any ideas from those two? Up to 105cm vs up to 125cm, and just RF vs both RF and FF?