what size are your tins? 4/2 i'd use for 6 inch tins....i always use stork for vicky sponges, if it is good enough for La Berry, it's good enough for me!
7 inch tins use 3 eggs
8 inch tins use 4 eggs
Use metal tins, I find silicone very very poor for baking, I do not think the heat transfers properly which means the cake is in the oven longer and dried out by the time it is cooked through
watch your oven is not too hot, i think 160 ish...i have a temperamental range so am not good at exact temps, and don't have it on a high shelf...again, this may be irrelevant if your oven is more modern than mine :o
I only use baking paper to line tins these days...greaseproof seems to have gone all weird and icky.
I also only mix by hand...big bowl, wooden spoon, I think it is very easy to over work the mix when the eggs and flour are in and doing by hand is gentler.
Also try and line teh sides as well as the bottom of the tins...it really helps to get a deep straight sided sponge, if the mix reaches the top edge of the tin it will tend to dome, if you have the sides lined it will continue to rise up.