I did a wee bit of digging too. It seems that there is a rebound effect for interferon too (and maybe Copaxone? It would certainly help to explain the terrible year I had after coming off it).
Anyway - there are a couple of studies of Tysabri rebound effect on the MSRC website. If you go to the following link, press Ctrl and f on your keyboard and then type rebound (into the little box) it will take you to the first rebound written in the text. Thereafter, each time you press enter it will take you to the next rebound in the text.
In summary, there seems to be a rebound effect, but it seems to vary with how long you’ve been on Tysabri (longer is more protective), how long you stay off it (longer is worse) and possibly how old you are (older is better). It may also depend on what you do while you’re off it - there is one study on there that suggests that taking an interferon may reduce the number of relapses however these patients were the ones with milder MS pre Tysabri, so it may well not be true for others. Both the studies that specifically looked at rebound were small studies so it’s hard to know how much faith to have in them. One thing is for sure: the only way not to have a rebound is to keep taking Tysabri, but that’s easy for me to say when I’m not JCV positive and on Tysabri 
This whole Tysabri “holiday” was never suggested by the people who make Tysabri - it started because some neuros decided it might be a good idea, without any data or research to back them up! I’m not sure that there is any research to say that taking a “holiday” actually protects against PML anyway, if you plan on going back to it after the break.
The fact you’ve been on Tysabri for more than 2 years may help protect you from rebound? The other thing to consider is how bad your MS was before Tysabri. If it wasn’t so bad, then perhaps an interferon would help to reduce any rebound effect? Absolutely no guarantees with this though.
If you come off Tysabri and you have a nasty relapse, will your neuro be able to get you back on it straight away? Will your neuro guarantee you regular (3 monthly max) MRI scans to check for new lesions? And if there are some, get you back on Tysabri?
This is probably a stupid question, but I’ll ask it anyway(!): have you ever had steroids? If you haven’t, then your risks of PML, even with JCV+, are much lower. (See Multiple Sclerosis Research: Natalizumab - Safety Update December 2011)
I hope this helps. Good luck with your decision.
Karen x