Does anyone have experience of making extra contributions to a UK pension, either through AVCs or another route, in order to compensate for years spent working in the US?
Presuming we're talking primarily about the UK state pension:
I've actually spoke to HMRC about this earlier this month, posted off form CF83, and waiting to officially hear back on costs and eligible years (I already know the years and the cost down to a few pounds, but I have to wait for HMRC's official 'offer'). The online HMRC page for NIC gaps (
https://www.tax.service.gov.uk/check-your-state-pension/account/nirecord/gaps) only shows the figures for (and only talks about) Class 3 contributions, If you worked in the UK before working in the US, you are probably entitled to make the (much cheaper!) Class 2 payments instead.
Before speaking to HMRC, I spent a couple of days modelling what the same amount of money would do invested in my other accounts, and concluded AVCs are almost always worth it. I had to use an after-inflation investment rate of 7% combined with an investment start date 20 years before pension age in order for Class 3 AVCs not to win out. I'd been posting to a some forums, and they pointed out that as I worked in the UK immediately before working in the US, I could make Class 2 AVCs (£160/year) instead of Class 3 (£800/year). At which point there was no point running any further models. Note: Class 2 is usually reserved for the self-employed, but not here!
Reading between the lines, it sounds like you may be thinking about taking money out of your US retirement funds to make AVC payments. If you have a small IRA / 401k and you want an excuse to shut it down to simplify the paperwork, fine, but otherwise, personally, I'd been doing whatever I could to find the money from a non-tax advantaged account.
While comparing growth with a tax-advantaged account was not a scenario I ran (I'm already maxed out), I'd personally do it to make Class 2 contributions. If only Class 3 was on the table, I'd think about it longer, but personally still do it, because it's a guaranteed return vs stock market probabilities.
Note that while you normally can only make payments for years within the last 6, due to the 2016 shake-up of the pension system, most folks can (until 5th Apr 2022) make payments back as far as tax year 2006-2007.
My personal worse-case assumption is that the UK State Pension will become means tested before I reach pension age, and that I (hopefully!) won't qualify for it. So I think of it more as an fallback thing. Your NIC history also controls you eligibility to a few other benefits (none that currently interest me), but it wouldn't surprise me to see that benefit list expand in the future.
If you worked in the US long enough to qualify for Social Security (10 years of paying in), having a UK state or company pension will almost certainly reduce you Social Security cheques due to a US regulation called WEP. This reduction only applies to pensions that the employer contributed to, which means historical months funded solely by AVC payments are not subject to WEP. While WEP can be as much as 50% of your in-scope UK pension(s), you'd be unlucky to get anywhere near that number. For example, in my case (13 years of existing NICs, 14 years of planned Class 2 AVCs, 8 years of future Class 3 AVCs, 13 years of max-capped US Social) the penalty worked out at $45.85 per week (£32.50 per week). Due to the... interesting... way WEP is calculated, the penalty does not increase with inflation (even if your UK pensions does) nor change with the exchange rate - it's dollar value is literally snapshotted once and fixed for all eternity (the only exception being if the UK pension is completely stopped).
While I'm dropping links:
edit: Fix broken link