I haven't been in 5.2 classes so I can't really tell if you're skill level is satisfactory enough to do Extension 1. It's already going to be "hard" for those doing 5.3 with topics such as Perms/Combs or Further Rates from experience however it never becomes boring at all, even with topics such as polynomials or inverse trig that seem to be at the easier side of the spectrum, they still are interesting topics as you gain a better understanding of the general topic (Functions and Trig) and a variety of techniques such as double angle formula or addition/multiplication of roots that can help you out in Advanced.
Ext-1 definitely takes up a significant amount of time that you could distribute to other subjects which people doing only Math-Adv take for granted however it is so worth it as it really assists your learning and understanding of math topics that much more. I recommend brushing up and mastering all basic and fundamental skills such as algebra, basic trig (sohcahtoa) and functions (parabola, linear graphs) and learning some of the content in 5.3 before doing the course over the holidays because it is a mistake made by many of my peers resulting in them with great amounts of stress and anxiety along with workload and leading them to either distribute an unhealthy amount of time in the schooldays refining their fundamental skills and completing the exercises or more frequently, just dropping Ext-1 altogether.
If you plan it out right, have a strong foundation in basic skills and you are enthusiastic then I don't see why you couldn't do Ext 1 but considering you're 5.2, you might need to put in more effort and dedication than your 5.3 counterparts to reaching a level competent to undertake the course REASONABLY (not overworking or spending too much time on Ext 1 solely)