Here is an excerpt from a dealer piece on the amount of negative convexity in the market. To translate,the dealer firm is estimating that if there was a parallel shift in the curve in which rates rise 25 basis points,that would be as if $ 149 billion 10 year notes appeared in the market place. Here is the relevant portion :
As far as the agency passthrough market is concerned, we are at the point of
maximum negative convexity at the moment (Figure 2). We estimate that 30-year
agency passthrough universe will extend by $149 billion 10-year Treasury
equivalents for a 25bps backup (parallel shift) in rates. Note that the change in duration of the 30-year passthrough universe for a 25 bps change in interest rates is a lot higher than that of corresponding MSRs, but only a small portion of negative convexity risk in agency passthroughs is actively hedged (only about 10%-15% of the risk) while almost all the negative convexity risk in MSRs is hedged fairly actively.