Front wheel's more important yes but you can only go so deep, a PX 101 is legal but right on the limit, a Zipp 1080 is just over and illegal. The deeper you go with a front wheel the more tricky the bike is to control in winds/gusts too which can counter the aero gain. The back isn't a problem as your weight on it keeps it stable.
Skinsuits do make more difference than you think Ivor. You're not going to go much quicker but in the second/£ ratio they score quite well, like the pointy hat.
As Sylv say though, if you ride with your knees out what's the point
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I think a lot of it is marginal gains though that you try and accumulate, same as focussing better, staying on the tri-bars all the time to keep the frontal area small etc. Personally I still think legs and head are the most important bits of kit you can own, put Hutch on a butcher's bike and he'll still be quick.