Do you know the maximum gene size that can be inserted into a lentiviral vector?
10-11Kb is about the maximum size that cen be inserted between the LTRs before you start seeing a reduction in packaging efficiency and therefore reduction in viral titers.
The reference below shows a logarithmic inverse relationship between packaging size and titer. Depending on where the sequence is inserted, you could see a drop of 1 log in titer for every 2Kb of insert.
Systematic Determination of the Packaging Limit of Lentiviral Vectors Mukesh Kumar, Brian Keller, Ndeye Makalou, Richard E. Sutton Human Gene Therapy. 2001, 12(15): 1893-1905.