Just because you've done well in Maths comps doesn't mean that you should explicitly do a subject, although with those result I would recommend you keep Maths Ext. 1. However, the first consideration should be that YOU want to do the subject. If you like Maths and want to do Maths Ext. 1, then you'll put effort in, most likely do well, and as a result get a slightly better UAI (slightly because MEX1 is just 1 unit, and in most cases a change in 1 unit won't greatly modify your UAI, unless the results are wildly different).
Anyways, as long as you do very well in whatever will replace Maths Extension 1, then your UAI shouldn't be greatly affected. However, with your results, I'd say keep it. If you can't, then make sure you work extra hard in History Extension (in your HSC year) to compensate.
I_F