As a general rule the PIE graph should not pass values of 280. I say general rule because the 280 limit is not really a fixed limit. The DVD-standard does allow this limit to be surpassed for a short time (called 'spikes'). In your case the values at position 3.3Gb and 3.6-3.8GB could still be considered as allowable spikes. However it looks that there is a rather huge out of specifications spike at position 2.5-3.1GB.
Practical experience has shown that disks with a constant PIE value of 500 often still play fine. Higher values do not necessarily make a disc unreadable. However there may be need to reduce the the read speed to as low as 2x. However very high values (above 500) may cause read errors. The amount of PIE that will cause read errors also depends on the drive used to read the disk. Some drives have better error handling than others.
A good quick test is the READ TRANSFER RATE TEST in Plextools. If this produces a smooth curve from 5x to 12x then the disk is likely to be good. At least this is proof that your writer does not have problems reading it.
In your case i'd recommend to do a PIF (SUM1 Test) scan. PIF values should not pass 4 (adjust scale in preferences to make such low values better visible). Values higher than 4 may cause fatal errors rather quickly. However an occasional single value of 5 or 6 may still be ok.