First, as noted, Tuilagi has played for the Lions at 12, he's played for England at 12, and he's beaten South Africa's disinterested reserves at 12 for his club. It really isn't as radical an idea as you'd like to make out.
Second, this is happening at some point as its the only way to get out 2 best centres - currently out 2 best centres by a big distance - into the team. It's definitely on the agenda, so why not sooner rather than later?
Third - either we have cemented Burrell with Ford and Joseph, or there is no time to cement a player next to them other than Burrell. I mean, think about it...
Fourth, everyone here advocates moving players all the time and only kicks off when they disagree with the individual move. I mean, Vunipola to 6, Itoje to 8, Daly to 12, Daly to cover wing just to pick a recent selection from this thread, people talking about Burgess before he's even moved there... basically, I think its more about you not rating Tuilagi as highly as you should and being puzzled that people do rate him higher than yourself. There's nothing logical about your objection otherwise.
I mean, if you think Tuilagi's gonna struggle to meet the bar set by our recent 12s, that's a pretty damning statement of him already...