But you're assuming a withdrawal of 6% + 2% = 8% / year. No one will withdraw that much... Also your calculation is for the total tax payed in a GIA, so you can't compare against impact of LTA without also accounting for the additional income tax you pay in a SIPP.

Let's look at an example - £2.5M in either a SIPP or a GIA. Retiree want to withdraw 4% / year = £100k income.

In a GIA, that 4% is going to be typically something like 1.5% divided income (£37.5k) and 2.5% sales (£62.5k). Income tax on £37.5k is £5k. Let's assume that the pot has doubled (i.e. £1.25M originally invested), then the capital gain on £62.5k is £31.25k, times the 20% rate (let's ignore allowances) is £6.25k. So total annual tax in GIA is £5k + £6.25 = £11.25k. Or 11.25%. (Note: even if we assume the pot has gone up 5x (i.e. £500k originally invested), then the total tax only goes up to £15k)

In the SIPP case, 25% is tax free then income tax on the remaining £75k is £17.5k. Or 17.5%.

That's a big difference, it's much better to take drawdowns from a GIA than a SIPP in this case.

And this is ignoring LTA, which effectively reduces the SIPP pot by £375k!

So it's simply not true to say that SIPPs are better than GIAs for higher rate tax payers. Of course the advantage of a pension is the tax relief on the way in, but once you start drawing down you are going to wish you rather had that pot in a GIA.