Regarding solid rear axle vs. IRS, the solid rear axle will quite often offer an advantage on the track, at the cost of trickier handling qualities. Road and Track repeatedly compared the f-body cars to the mustangs and although the new IRS mustangs handled "better" than the f-body in terms of ease of driving over bumps and uneven pavement, the f-body still soundly trounced the mustang even when the mustangs finally got a horsepower advantage.
Go to ANY SCCA autocross event, and compare the performance of stock mustangs against stock f-bodies... If you can find any stock mustangs competing that is. The f-body cars simply destroy the mustangs on the track so almost nobody races mustangs in SCCA stock classes and nobody actually wins driving them either.
The only drawback to the solid rear axle is axle hop under acceleration (controllable with suspension tweaks), axle hop under braking (controllable by skilled driving and/or suspension tweaks), and quirky handling when accelerating or cornering over uneven pavement or bumps. Even then, the solid rear axle will often come out with higher overall performance than the IRS.
Almost forgot... You can get a bulletproof solid rear capable of handling a LOT more power and torque than the stock IRS, and to my knowledge they don't sell an IRS upgrade capable of handling as much power as commonplace solid rear ends. You can spend only a couple thousand bucks and get a solid rear end that can handle well over 1000 hp. I don't know if there is an IRS swapout that can take that much power but if there is, there is no way it is nearly as cheap.
Track results do not lie... Yea everyone WISHES someone would develop an IRS that matched the last generation of solid axles in terms of overall grip and track performance, but it hasn't happened in US auto designs. Most car designers realize that the last bit of performance isn't worth the handling tradeoff so they go with a more comfortable and predictable IRS wherever feasable, but again track results show that the solid rear axle typically beats the IRS in comparable cars.