The most tax efficient use of AVC's is to bridge any shortfall between the Revenue max retirement lump sum and whatever lump sum the Pension Scheme will pay. If effect you get tax relief (at up to 40%) on the way in and get it all back (hopefully) tax free when you retire. If you do this on the run-in to retirement then it is a very good deal. BUT, you need to check the numbers. So if when you retire you are getting full service benefits (based say on 40 years service), you need to be sure that there is a gap to be bridged???
Investing AVC's beyond funding for a lump sum shortfall, is questionable. There is no tax advantage if you are going to be a top rate tax payer in retirement - i.e. 40% relief on contributions but 40% +USC on any additional pension income.
So yes, do maximise your tax-free lump sum, but beyond that its probably not worthwhile (unless you will drop to a 20% tax rate in retirement).
Finally, if investing for additional tax-free lump sum over a short period I would only invest in a Cash Fund. Any other fund involves some investment risk which makes little sense sine the tax-relief represents you real (best) investment return.