If you then want to sell some and transfer the gain into a sipp to get the 20% tax relief (or highest band of income tax you pay), then yes, that is a tax efficient flex. Although then have the restriction of when you can access your SIPP
Discussion
Of course but as she wants to have a SIPP anyway. Rather than paying direct into the SIPP this could be a way to boost gains and be tax efficient. Ie just delay the amount into the SIPP until the end of the year and hopefully get gains in the isa that are bigger than what would have gone in originally. Then also you are not tying all capital up in a SIPP but having some left in isa to use some other time. I guess it only works if the isa goes up a lot but with MSTR this seems possible.