I believe you'll be taxed and IIRC pay a 10% penalty on any earnings that occur within the non-deductible T-IRA between the time of your contribution and the recharacterization to the Roth IRA. So either (a) put the investment inside the T-IRA in something that doesn't earn much, (b) be ready to pay taxes on any growth, and/or (c) do the recharacterization step as rapidly as possible.
I thought the IRS clarified this in 2014/5 and your earning can just got to a traditional IRA?
That's for the MEGABackdoor Roth. This is a Backdoor Roth. And actually, there isn't a 10% penalty when you convert (not recharacterize on this one - convert). Just taxes on any growth.
One gotcha OP - do you have any traditional IRA balance out there? If so, you need to get that moved to a 401K before you do this, or revisit this plan.
The conversion will be pro-rated, so if you've got say $10K out in a tIRA already, and you do a backdoor Roth for $5K, you'll owe taxes on 2/3 of your conversion. Probably not the result you're expecting.